Scientific nomenclature of a species must have, at least, two names: one that classifies it as genus and the other that identifies it as species. The name related to genus is the first and must begin in uppercase, the other following names must be written in lowercase.

Besides this rule, scientific names of species must stand out and be written either in italics or underlined or still bolded or between quotation marks.

For example, the scientific name of the human species is "Homo sapiens", indicating that it belongs to the genus Homo.

Scientific nomenclature of species is important because it universalizes the way to refer to a species making it easier for people of different languages and cultures to understand each other. Same species that have very different names in different regions of the planet can be identified easily by their scientific binomial name.

SARS is caused by a virus from the coronavirus group, a RNA virus (retrovirus). SARS can be fatal.

The increase in product formation in the chemical equilibrium of the formation of bicarbonate from carbon dioxide and water heightens the concentration of hydrogen ions and thus lowers the pH of the solution.

Type I diabetes, also known as juvenile diabetes, or insulin-dependent diabetes (this name is not adequate as type II diabetes may become insulin- dependent), is the impaired  production of insulin by the pancreas believed to be caused by destruction of cells of the islets of Langerhans by autoantibodies (autoimmunity). Type II diabetes occurs in the adult individual and it is often diagnosed in people of more advanced age. In type II diabetes there is normal or low secretion of insulin by the pancreas but the main cause of the high glycemia is the peripheral resistance of the cells to the action of the hormone.

During the day the transpired volume of water is higher than the volume absorbed by the roots. At night the situation reverses and the roots absorb more water than the transpired volume. It is observed that the transpired and the absorbed volumes practically equal in a day.

The energy flux along a food chain is always unidirectional, from the producers to the decomposers. 

The five kingdoms of living beings are the kingdom Monera, the kingdom Protista, the kingdom Fungi, the kingdom Plantae and the kingdom Animalia.

Viruses are out of this classification and sometimes they are said to belong to their own kingdom, the kingdom Virus.

Birds are terrestrial animals but the majority of species also explore the aerial environment by flying.

The pulmonary respiration is controlled by the neural respiratory center located within the medulla (the lower part of the brain continuous to the spinal cord).

Diabetic patients are often advised to ingest less carbohydrates since these substances are degraded into glucose and this molecule is absorbed in the intestines. The dietary sugar restriction goal is to control glycemia to maintain it at normal levels.

The cells that constitute the xylem ducts are dead cells killed by lignin deposition. The cells of the phloem are living cells.

The chain concept is a theoretical model to study the energy flux in ecosystems. Actually in an ecosystem the organisms are part of several interconnected food chains, forming a food web. Therefore the chain is a theoretical linear sequence and the web is a more realistic representation of nature in which the food chains interconnect forming a web. 

Cellular beings are divided into two groups: the prokaryotes, unicellular beings whose sole cell does not have a delimited nucleus, and eukaryotes, uni or multicellular beings with cells having delimited nucleus.

Birds, like every vertebrate, have a closed circulatory system. The heart is similar to the mammalian heart, having four chambers (two atria and two ventricles) and with no mixture of venous and arterial blood. (In mammals, however, the aorta curves down to the left and in birds it curves down to the right).

The shifting of the chemical equilibrium of the formation of bicarbonate from carbon dioxide and water into the reverse reaction (production of water and carbon dioxide) means spending of hydrogen ions and thus it increases the solution pH.

Under normal conditions the glucose filtered by the renal glomeruli is almost entirely resorbed in the nephron tubules and not excreted in urine. With the elevated glucose blood level the renal tubules cannot resorb all the filtered glucose and some amount of the substance appears in the urine. This amount is enough to provide the sweet taste that helped Hypocrates to diagnose diabetes and to differentiate it from other diseases accompanied by polyuria. Nowadays the method is inconceivable due to the danger of contamination of the tester by disease agents possibly present in the patient's urine.

Lignin is important because it is deposited on the cell wall of the xylem cells providing impermeability and rigidity to the xylem vessels.

Trophic levels correspond to positions on a food chain. Therefore producers always belong to the first trophic level and decomposers to the last trophic level, consumers that directly eat the producers belong to the second trophic level and so on.

There is no limit regarding the number of trophic levels on a chain, since many orders of consumers can exist.

The kingdom Animalia is the animal kingdom. Commonly the kingdom Animalia is subdivided into invertebrates and vertebrates.

SARS is caused by a virus from the coronavirus group, a RNA virus (retrovirus). SARS can be fatal.

Acidosis is the condition in which the blood pH is abnormally low. Alkalosis is the condition in which the blood pH is abnormally high. Normal pH levels for the human blood are between 7.35 and 7.45 - slightly alkaline.

The general goal of the diabetes treatment is to maintain normal glycemic levels. Type I diabetes is treated with parenteral administration of insulin. Insulin must be administered intravenously or intramuscularly because as a protein it would be digested if ingested orally. In type II diabetes treatment is done with oral drugs that regulate the glucose metabolism or in more severe cases with parental administration of insluin. The moderation of carbohydrate ingestion is an important aid to diabetes treatment. The diabetes treatment with the use of hypoglycemic agents, like insulin or oral medicines, must be carefully and medically supervised since if wrongly used these drugs may abruptly decrease the glucose blood level, cause hypoglycemia and even death. Many other forms of diabetes treatment are under research worldwide.

Root pressure is the pressure that forces water from the soil to be absorbed by the xylem of the root. It is due to the osmotic gradient between the interior of the root and the soil.

The three types of trophic pyramids studied in Ecology are the numeric pyramid, the biomass pyramid and the energy pyramid.

Generally the variable dimension of the pyramid is the width, and the height is always the same for each represented strata of living beings. The width therefore represents the number of individuals, or the total mass of these individuals or the available energy in each trophic level.

The kingdom Monera is the kingdom of the prokaryotes, composed of bacteria and archaebacteria.

Wings associated to a well-developed pectoral musculature, pneumatic bones, less accumulation of feces in the bowels due to the absence of the colon, absence of the bladder (no urine storage), aerodynamic body and lungs with specialized air sacs are all adaptations which enable birds to fly.

The pulmonary ventilation frequency (number of inhalations per time unit) rises or lowers the carbon dioxide concentration in blood. If it is intense the gas is more eliminated to the exterior and if it is reduced the gas is retained inside the organism. Applying the principles of chemical equilibriums to the formation of bicarbonate from carbon dioxide and water one gets the following: if the carbon dioxide concentration is increased the equilibrium shifts towards the formation of bicarbonate and liberation of hydrogen ions and the pH  of the solution is lowered; if the carbon dioxide concentration is lowered the equilibrium shifts reversely towards the formation of water and carbon dioxide and also of more hydrogen ions spending and the pH of the solution is raised.

Bacteria do not naturally synthesize insulin. It is possible however to implant human genetic material containing the insulin gene into the bacterial DNA. The mutant bacteria then multiply and produce human insulin. The insulin is isolated and purified for later commercialization. This biotechnology is known as the recombinant DNA technology. Besides human insulin the pharmaceutical industry also produces insulin to be used by humans made from the pancreas of pigs and cows.

Capillarity is the phenomenon through which water moves inside extremely thin tubes (capillaries) aided by the attraction between water molecules and the capillary wall. The capillarity phenomenon is possible because water is a polar molecule and forms intermolecular hydrogen bonds. 

Therefore there is electrical attraction (adhesion force) between the capillary wall and the water molecules that then pull each other (cohesion force) since they are bound. Not just water but other liquids may move inside capillaries by capillarity.

Capillarity is not too relevant for the transport of water in plants. It contributes only to a few centimeters of ascension.

The kingdom Protista comprises protozoans and algae.

Birds are uricotelic, i.e., like reptiles, they excrete uric acid. This substance needs less water to be eliminated and so it helps to reduce the body weight thus aiding in flight.

If the body experiences acidosis the respiratory center located in the medulla gets the information and induces the increase of the respiratory frequency. The increment of the respiratory frequency makes the body eliminate more carbon dioxide and to shift the equilibrium of the formation of bicarbonate towards the spending of more hydrogen ions and thus the blood pH raises.

Each adrenal gland is located on the top of each kidney (forming a hat-like structure for the kidneys), so there are two glands. The adrenal parenchymal structure is divided into two portions: the most peripheral is the cortical portion, or adrenal cortex, and the central is the medullary portion, or adrenal medulla.

Water enters the roots due to the root pressure and a water column is maintained within the xylem from the roots to the leaves. The most important factor that makes water ascend is transpiration, mainly in the leaves. As leaves lose water by transpiration their cells tend to attract more water creating suction inside the xylem. The cohesion property of water that keeps its molecules bound (one pulls the other) by hydrogen bonds helps the process.

Numeric pyramids represent the number of individuals in each trophic level of a food chain. 

The nine phyla of the animal kingdom are: Porifera (poriferans), Cnidaria (cnidarians), Platyhelminthes (flatworms), Nematoda (roundworms), Annelida (annelids), Mollusca (molluscs), Arthropoda (arthropods), Echinodermata (echinoderms) and Chordata (chordates).

Crystallization is the process of transformation of viral components into organized solid particles.

Crystallization of biological macromolecules, including viral components, is used to study structural characteristics, for example, through X- rays, laser beams, etc.

If the body undergoes alkalosis the respiratory center located in the medulla gets the information and induces the lowering of the respiratory frequency. The reduction of the respiratory frequency makes the body retain more carbon dioxide and to shift the equilibrium of the formation of bicarbonate towards the production of more hydrogen ions and thus the blood pH lowers.

Malpighi’s girdling, or tree girdling, is the removal from a stem of a complete external girdle containing the phloem (that is more external) but preserving the xylem (that is more internal).

When a girdle like that is removed below the branches the plant dies because organic food (sugar) is disallowed to pass to the region below the girdle and thus roots die from the lack of nutrients. Since roots die the plant does not get water and mineral salts and dies too.

If an intermediate level of a numeric pyramid has its variable dimension decreased, i.e., if the number of individuals of that level is reduced, the number of individuals of the level below will increase and the number of individuals of the level above will be reduced. That happens because the individuals of the level below will face less predators and the individuals of the level above will have less available food. 

The kingdom Fungi is formed by fungi.

Birds have lightweighted bones with internal spaces filled with air. These bones are called pneumatic bones. This feature reduces the corporal density of the animal facilitating the flight.

Coleoptile is the first (one or more) aerial structure of the sprouting plant that emerges from the seed. It encloses the young stem and the first leaves, protecting them.

The top of the coleoptile generally is the region where auxins are made. If this region is removed, plant growth stops since auxins are necessary to promote growth and tissue differentiation.

In a numeric pyramid the base corresponds to the first trophic level, i.e., to the producers. The top level of the pyramid corresponds generally to the last consumer order of the food chain (since the number of individual decomposers, most of them microorganisms, is too large to be represented). 

Chordates are divided into protochordates (cephalochordates and urochordates) and vertebrates.

Regarding external coverage, birds are similar to reptiles as they present impermeable keratinized coverages.

Concerning reproduction, in both fecundation is internal and the embryo develops within a shelled egg.

Regarding excretion, both excrete uric acid.

The testicles make androgenic hormones, the main of them being testosterone. The ovaries produce estrogen and progesterone.

Plant hormones, also called phytohormones, are substances that control the embryonic development and the growth of the adult plant.

Biomass pyramids represent the sum of the masses of the individuals that participate in each trophic level of a food chain.

The kingdom Plantae is composed of plants.

Algae are classified into the kingdom Protista and not into the kingdom Plantae (they are not plants).

The eggshell is basically made of calcium carbonate. The white, or albumen, is composed by albumin, a protein. The yolk is predominantly constituted of lipids but it also contains proteins and vitamins.

Circulation is the movement of substances like nutrients and gases within blood vessels and cavities throughout the organism.

The placenta is not a permanent gland of the endocrine system but it also has endocrinal function. The placenta produces estrogen and progesterone. It also secretes human chorionic gonadotropin (HCG, that acts similarly to the hypophyseal LH), human placental lactogen, similar to prolactin and stimulant of the mammary glands, and a series of hormonal peptides similar to the hormones of the hypothalamus-hypophysis axis.

Indolacetic acid (indolyl-3-acetic acid), or IAA, is the main natural auxin made by plants. It promotes plant growth and cellular differentiation.

Since the numeric pyramid represents the quantity of individuals in each trophic level of the food chain, inferior trophic levels with less individuals than the superior trophic levels may exist. For example, a single tree can serve as food to millions of insects. 

Chordates are divided into protochordates (cephalochordates and urochordates) and vertebrates.

Birds, like every vertebrate, have sexual reproduction. Their embryos develop within shelled eggs containing extraembryonic membranes and outside the mother’s body.

Birds copulate. Fecundation is internal and it occurs only before the female gamete is involved by the calcareous eggshell.

The circulatory systems can be classified into open circulatory system and closed circulatory system.

The placenta is not a permanent gland of the endocrine system but it also has endocrinal function. The placenta produces estrogen and progesterone. It also secretes human chorionic gonadotropin (HCG, that acts similarly to the hypophyseal LH), human placental lactogen, similar to prolactin and stimulant of the mammary glands, and a series of hormonal peptides similar to the hormones of the hypothalamus-hypophysis axis.

The main natural plant hormones and their respective functions are the following:

Auxins (the best known natural auxin is IAA, indoleacetic acid): their function is to promote plant growth, distension and cellular differentiation. Gibberellins: have action similar to auxins (growth and distension), stimulate flowering and fruit formation and activate seed germination. Cytokinins: increase cellular division rate and together with auxins help growth and tissue differentiation, slow the plant aging. Ethylene (ethene): a gas released by plants that participates in the growth process and has noteworthy role in fruit ripening and in leaf abscission.

When biomasses are compared often the concept of dry mass is used. The dry mass is the total mass less the water mass of an individual. The total mass is also called fresh mass. To use dry mass instead of fresh mass is useful because among living beings there are differences related to the proportion of water within their body and such differences can distort the quantitative analysis of incorporated organic material.

Vertebrates are different because they have a spinal column (vertebral column). In these animals the  notochord of the embryonic stage is substituted by the vertebral column in adults.

Reptiles are heterothermic, i.e., they do not control their body temperature.

Birds however are the first homeothermic animals, they are able to maintain their body temperature constant.

There are many birds that live in intense cold regions. Penguins are examples of birds that live in polar region.

Not all animals have a circulatory system. Poriferans, cnidarians, platyhelminthes and nematodes (in these there are the pseudocoelom fluid but no vessels) are avascular animals. Echinoderms do not have true circulatory systems either.

Patients with transplanted organs are prone to host versus graft rejection since their own immune system tends to attack the grafted organ because of recognition of the grafted tissue as foreign matter. In the prevention and treatment of this common problem patients are given glucorticoids or other immunosupressants. Glucocorticoids have an immunosuppressant action and so they reduce the aggression of the immune system against the graft.

The immune action however is also very important for the individual. The immune system defends the body against invasion and infection by pathogenic agents (virus, bacteria, toxins) besides being fundamental for the elimination of modified cells that may proliferate and cause cancer.

Patients receiving immunosuppressants like glucocorticoids are thus under increased risk of infectious and neoplastic diseases.

Synthetic auxins, like indolebutyric acid (IBA) and naphthalenic acid (NAA) are substances similar to IAA (a natural auxin) but artificially made. Some are used to accelerate methods of asexual reproduction (like grafting or budding) and others are even used as herbicides since they selectively kill some plants (mainly dicots).

Energy pyramids represent the amount of available energy in each trophic level of the food chain.

The five classes of vertebrates are: fishes (osteichthyes and chondrichthyes), amphibians, reptiles, birds and mammals. Humans classify as mammals.

Zoonoses are human diseases transmitted by animals. Psittacosis, a bacterial disease, hystoplasmosis and cryptococcosis, fungal diseases, are examples of zoonoses transmitted by birds.

Open circulatory system is the one in which blood does not circulate only inside blood vessels but it also falls in cavities that irrigate tissues. In the open circulatory system the blood pressure is low and generally the blood (called hemolymph) has low cellularity. Arthropods, molluscs (the cephalopods are exception) and protochordates have open circulatory system.

The immune system performs specific defense against agents, the antigens, that are foreign or harmful to the body. Exogenous antigens are often in contact with the skin or entering the airway, the digestive tube and the genital orifices and mucosae. They can also penetrate the circulation directly through wounds.

Auxins are produced and found in large amount in the apical buds of the stem and shoots and in the young leaves.

The luminous energy used in photosynthesis is transformed into chemical energy.

The embryonic development is direct, there is no larval stage.

A closed circulatory system is one in which blood circulates only inside blood vessels. For this reason the blood pressure is higher in animals with closed circulatory system. The cellularity of the blood is also higher with many specific blood cells. The closed circulatory system is a feature of annelids, cephalopod molluscs and vertebrates.

Inflammation is the initial response of the unspecific defense system versus aggressions against the body (the aggressions may be caused by infectious parasites, chemical contamination, trauma, physical agents like heat and fire, autoimmunity, etc.). During inflammation a series of unspecific leukocytes present in the circulation are attracted to the injury site in an attempt to destroy harmful agents and to isolate the affected region of the tissue.

Parthenocarpic fruits are those produced without fecundation. Some plants naturally make parthenocarpic fruits, like the banana tree, stimulated by their own hormones.

Angiosperms that do not naturally produce parthenocarpic fruits may do it if auxins are applied to flowers before fecundation. Therefore even without fecundation the ovaries grow and fruits are formed although seedless.

Gross primary production of an ecosystem, or GPP, is the quantity of organic material found in a given area in a given period.

Since only autotrophs produce organic material and photosynthesis is the main production process, GPP is a result of the photosynthesis.

The phylum Porifera contains the simplest creatures of the animal kingdom. Sponges are aquatic sessile beings (they are not able to move by themselves and they keep themselves attached to substrates), they do not have tissue diversity and their bodies have pores (feature after which their name comes).

They are multicellular, like all beings of the animal kingdom.

Examples of representing beings: chickens, sparrows, parrots, ostriches, penguins. Basic morphology: aerodynamic body, feathers, pneumatic bones, horny beaks. Skin: impermeable keratinized, feathers, uropygial gland.

Respiration: pulmonary. Circulation: closed and complete, heart with four chambers. Nitrogen waste: uric acid. Thermal control: homeothermic. Types of reproduction: sexual, internal fecundation, shelled eggs with extraembryonic membranes.

In animals that do not present the circulatory system the transport of substances occurs by cell to cell diffusion. The blood is a fundamental means of substance transport for larger animals since in these animals there are tissues distant from each other and from the environment thus making diffusion impossible.

Apical dominance is the phenomenon through which high (over the positive range limit) auxin concentrations due to auxins from the apical bud moving downward the stem inhibit the growth of the lateral buds of the plant. At the beginning of the stem development the apical dominance causes the plant growth to be longitudinal (upwards) since the growth of the lateral buds remains inhibited. As the lateral buds become more distant from the apex the auxin concentration in these buds lowers and shoots grow more easily.

The growth of tree branches can be stimulated preventing the apical dominance through the removal of the apical bud.

A superior trophic level always has less available energy than inferior trophic levels. This is because in each trophic level only a fraction of the organic material of the level below is incorporated into the consumers (into their bodies). The other part is eliminated as waste or is used in the metabolism as energy source. Therefore it is never possible to have energy pyramids with inverted conformation, i.e., with the tip to the bottom and the base to the top. It is also not possible to have superior trophic levels with a variable dimension larger than inferior ones. In every energy pyramid, from the base to the top, the size of the variable dimension decreases.

Sponges are filtering beings. They make water enter their bodies by their lateral pores. Water then circulates inside the central cavity and exits through the osculum.

The typical features of mammals are: body (more or less) covered with hair; presence of the diaphragm muscle (that separates the thorax from the abdomen); mammary glands that produce milk (in females); enucleated blood red cells; middle ear with three ossicles.

The closed circulatory system is more efficient. Since blood circulates only inside blood vessels it can do it with more pressure reaching farther distances between the organs where hematosis happens and the peripheral tissues. In addition the circulatory speed also heightens making possible more oxygen supply to great consuming tissues, like the muscle tissues that then can perform faster movements. Animals with an open circulatory system (with the exception of insects that do gas exchange independently from the circulation) are generally slower and have a low metabolic rate.

When some tissue injury occurs histamine and other vasoactive substances (called mediators of inflammation) are released, they cause vasodilation and the blood flow to the affected site increases. Granulocyte leukocytes present in the blood are attracted to the site of the injury by substances known as chemotactic factors also released by the injured tissue and by the active granulocytes in the area. The granulocytes exit the capillaries by diapedesis, i.e., using pseudopods. Macrophages present in the region are activated too. These cells flood the extracellular space of the affected area trying to kill or eliminate harmful agents, to prevent tissue  necrosis and to isolate the damaged tissue.

Auxins stimulate the formation of conductive vessels, xylem and phloem, promoting the thickening of the stem.

Mainly water and light, but also mineral salts, temperature and carbon dioxide are factors that interfere with the gross primary productivity. 

Sponges live exclusively in an aquatic environment and they are attached by their base to a substrate (fixation ground). Sponges are filtering animals, they nourish themselves from nutrients that enter their atrium brought in with water.

The three groups into which mammals are divided are: monotremes (or prototherian, e.g., platypus), marsupials (or metatherian, for example, kangaroos) and placental (or eutherian, such as humans).

In insects the circulatory system is open but this system does not participate in the gas exchange process and in oxygen supply to the tissues. Gases go in and out through the independent tracheal system that allows direct contact of cells with the ambient air. Therefore an insect can supply the great oxygen demand of its fast-beating wing muscles even having open circulatory system.

Pus is a residual of the inflammatory reaction. It contains a mixture of fragments of dead leukocytes, infectious agents (generally bacteria) and tissues.

In some parts of the plant (stem, roots, lateral buds) there are auxin concentration ranges in which the hormonal action is positive (stimulate growth). It is observed that concentrations over the superior limit of those ranges have the opposite effect (inhibition of growth).

Part of the organic material synthesized by the producers is consumed as energy source for the metabolism of the own producer individual. The other part is incorporated (into the body) and becomes available to heterotrophic beings of the ecosystem. In each following trophic level part of the organic material is used in the metabolism of the individuals of the level, the other part is eliminated as waste and only a fraction is incorporated and becomes available as food for the following level. 

Sponges can close their pores to avoid the entrance of water into their bodies in the presence of stimulus that may mean danger. This method however is rudimentary but it is actually a protection attempt against nocent agents.

Mammals of the monotreme group (platypus, echidnas) are oviparous, egg- laying, and they do not have a placenta. Mammals of the marsupial group (kangaroos, koalas, opossums) do not have a placenta either; females of this group give birth to embryonic young that then continue development within the mother's pouch. Placenta only forms in female placental mammals.

The typical components of the closed circulatory system are the blood vessels within which blood circulates (arteries, veins and capillaries), a pumping organ (heart) and the blood or bloodlike fluid.

In the tissue region where inflammation occurs bacterial toxins, cytokines, prostaglandins, interleukins and endothelins are released. These substances gain the circulation and reach the central nervous system which then commands the increase of the body temperature.

Gibberellins are plant hormones that stimulate plant growth, flowering and fruit formation (also  parthenocarpy) and the germination of seeds. There are more than 70 known types of gibberellins. Gibberellins are made in the apical buds and in young leaves.

Net primary production is the gross primary productivity less the organic material consumed as energy source in the metabolism of the producers: NPP = GPP – (organic material spent in aerobic respiration). It represents the organic material available in the first trophic level.

The base of the energy pyramids must represent the NPP and not the GPP since the idea of these pyramids is to show the available energy in each trophic level of the food chain. 

Sponges have bodies in the form of tubular vases or globes open in the upper extremity. They have an internal central cavity and porous walls. The central cavity is called spongocoel and the opening in the upper extremity is called osculum.

The orders into which placental mammals are divided are the following:

Artiodactyls, mammals with an even number of fingers in claws or paws like, e.g., cows, sheep, giraffes. Carnivorous, predators with canine teeth like dogs, lions, tigers. Cetaceans, aquatic animals without posterior limbs and similar to fishes, like whales and dolphins.

Edentates, creatures with rare or absent teeth, like sloths, armadillos, anteaters. Lagomorphs, small-sized mammals having three pairs of continuously growing incisive teeth specialized in gnawing, like rabbits and hares.

Perissodactyls, also known as ungulates (hooved), big-sized animals with an odd number of fingers in each paw, e.g., horses and rhinos. Primates, characterized by the big cranium and well-developed brain, like humans and apes. Proboscideans, big-sized animals whose nose and superior lip form the trunk (snout), e.g., elephants.

Chiropterans, flying nocturnal mammals (bats). Rodents, animals with two pairs of continuously growing incisive teeth, e.g., mice, rats, castors, squirrels.

Sirenians, aquatic mammals of freshwater, deprived of posterior limbs, like dugongs and manatees.

Cephalopod molluscs, like octopuses and squids, have a closed circulatory system with blood pumped under pressure flowing within vessels. Bivalve molluscs, like mussels and oysters, have an open circulatory system (also known as lacunar circulatory system) where blood flows under low pressure since it falls in cavities of the body and does not only circulate within blood vessels. Molluscs with closed circulatory systems are larger, agile and can actively move; molluscs with open circulatory systems are smaller, slow and some are practically sessile.

The main leukocytes that generally multiply and participate in the inflammation reaction against bacterial infections are the neutrophils. In this type of inflammation the blood level of  these cells are increased, a clinical condition known as neutrophilia. In the bacterial inflammation fragments of dead bacteria, dead neutrophils and tissues form the pus.

Physical and chemical environmental factors, like intensity and position of light in relation to the plant, gravitational force, temperature, mechanical pressures and chemical composition of the soil and of the atmosphere, can also influence the growth and development of plants.

The water cycle represents the circulation and recycling of water in nature. Liquid water on the planet surface is heated by the sun and turns into water vapor that gains the atmosphere. In the atmosphere large volumes of water vapor form clouds that when cooled precipitate liquid water as rain.

Therefore water comes back to the planet surface and the cycle is completed. As possible steps of the cycle, water may still be stored in subterranean reserves or in the form of ice in mountains and oceans and it may also be used in the metabolism of living beings, incorporated into the body of the individuals or excreted through urine, feces and transpiration. 

Sponges have their outer wall covered by flat cells called pinacocytes and having pores well-delimited by special cells called porocytes. The internal wall is filled with choanocytes, flagellate cells specialized in phagocytosis of food brought to the central cavity; the choanocyte flagella also maintains the water flux inside the sponge.

Between the outer and the inner coverage of the poriferan body there are cells with amoeboid movement (by pseudopods), the amoebocytes; since they are embedded in connective matrix, amoebocytes move and distribute nutrients to other cells and they also produce spicules that like a primitive skeleton fill the tissue and support the body structure. (Some poriferans have an internal skeleton, an endoskeleton, made of spicules and organic fibers.)

Mammals breathe through lungs, their respiration is pulmonary.

The heart is a muscular organ that contains chambers (right atrium and right ventricle and left atrium and right ventricle) through which blood passes. The blood enters the heart in the atria, goes to the ventricles and then leaves the organ. The blood is pumped out of the heart by the contraction of the muscle fibers that form the ventricular walls. The contraction reduces the ventricle volume thus increasing the internal pressure and the blood then flows to the exit vessels (pulmonary artery for the right ventricle and aorta for the left ventricle). When ventricular muscle fibers distend the ventricles regain their original size and receive new blood flow coming from the atria.

The main leukocytes that generally multiply and participate in the defense against worm infections are the eosinophils. In this type of inflammation the blood level of these cells are increased, a clinical condition known as eosinophilia. Eosinophils are also increased in allergic conditions.

Cytokinins are phytohormones active in the promotion of cellular division, they slow down the aging of tissues and act together with auxins stimulating plant growth. Cytokinins are produced by the root meristem and distributed through the xylem.

Biogeochemical cycles are representations of the circulation and recycling of matter in nature. The main biogeochemical cycles studied in Ecology are the water cycle, the carbon cycle and the nitrogen cycle. 

Sponges are different from other animals since they present only intracellular digestion. They do not have a digestive system nor do they release digestive enzymes in the spongocoel to cause extracellular break down of nutrients.

Mammals present a closed and complete circulatory system. The heart has four chambers and the arterial blood does not mix with venous blood.

Venous vessels are every blood vessel that carries blood from the tissues to the heart. Veins and venules are venous vessels. Venules are thin veins that are continuous to capillaries. In general venous vessels carry venous venous blood. The pulmonary veins that carry blood from the lungs to the left atrium of the heart however contain arterial blood.

The main leukocytes that generally multiply and participate in the defense against viral infections are the lymphocytes. In this type of inflammation the blood level of these cells are increased, a clinical condition known as lymphocytosis.

Tropisms are movements caused by external stimulus. In Botany the studied plant tropisms are: phototropism (tropism in response to light), geotropism (tropism in response to the earth gravity) and thigmotropism (tropism in response to mechanical stimulus).

The sun can be considered the motor of the water cycle because upon its energy the transformation of liquid water into water vapor depends. So the sun is the energy source that causes water to circulate in nature. 

Apart from sponges, that do not have a digestive cavity where extracellular digestion takes place, all other animals have a digestive system with an internal cavity in which extracellular digestion occurs.

Like chondrichtian fishes and adult amphibians, mammals are ureotelic, i.e., they excrete urea.

Systole and diastole are the two stages into which the cardiac cycle is divided. Systole is the stage when the contraction of ventricular muscle fibers occurs and the ventricles are emptied. Diastole is the stage of the cardiac cycle when the ventricular muscle fibers distend and the ventricles are filled with blood.

If the inflammatory attack is not enough to halt the infectious process the body still relies on a specific defense, the immune response proper (humoral and cellular) performed by the lymphocytes.

The plant hormone notable for stimulating and accelerating fruit ripening is the gas ethylene (ethene). By being a gas, ethylene acts not only in the plant that produces it but also in neighboring ones.

Some fruit processing industries use ethylene to accelerate fruit ripening. On the other hand, if the intensification or acceleration of fruit ripening is not desirable care must be taken to prevent the mixture of ripe fruits that release ethylene with the others.

Water is the main solvent of living beings and it is necessary practically for all biochemical reactions, including as reagent of photosynthesis. Many properties of water are very important for life.

Carbon is the main chemical element of organic molecules; carbon dioxide is also reagent of photosynthesis and product of the energetic metabolism of living beings.

Nitrogen is a fundamental chemical element of amino acids, the building blocks of proteins that in their turn are the main functional molecules of living beings; nitrogen is also part of the nucleic acid molecules, the basis of reproduction, heredity and protein synthesis.

The gas exchange in sponges happens by diffusion from the exterior to the cells that absorb molecular oxygen and liberate carbon dioxide.

Placental mammals reproduce sexually, they have internal fecundation and they are viviparous, i.e., their embryo develops within the mother’s body and from her it gets the nutrients through the placenta.

Arterial vessels are every blood vessel that carries blood from the heart to the tissues. Arteries and arterioles are arterial vessels. Arterioles are thin arteries that end in capillaries. Not all arteries however contain arterial blood (highly oxygenated blood). The pulmonary artery and its ramifications, arteries that carry blood from the right heart ventricle to the lungs, contain venous blood.

Humoral specific immune response is the defense system by means of antibodies, defense proteins secreted by lymphocytes that attack foreign agents with high specificity. Cellular specific immune response is the defense system by means of specific lymphocytes (cells) that directly attack other foreign cells and agents.

Whenever one side of a stem, branch or root grows more than the other side the structure curves towards the side that grows less. (This is an important concept for plant tropism problems).

Sponges do not have a nervous system neither circulatory system nor excretory system.

Fecundation in mammals is internal, with copulation. In the contemporary world human technology is able to promote artificial external fecundation of human gametes and of gametes of other animals.

Only capillaries perform exchange of gases and other substances with the tissues.

Antigen is any substance, particle or infectious agent recognized as foreign to the body. The contact of the antigen with the body promotes a defense reaction against the antigen (unspecific, specific or both).

Phototropism is the movement of plant structures in response to light. Phototropism may be positive or  negative. Positive phototropism is that in which the plant movement (or growth) is towards the light source and negative phototropism is that in which the movement (or growth) is inverse, away from the light source.

Phototropism relates to auxins since the exposition of one side of the plant to light makes these hormones concentrate in the darker side. This fact makes the auxin action upon the stem to be positive, i.e., the growth of the darker side is more intense and the plant arcs towards the lighter side. In the root (when submitted to light, in general experimentally) the auxin action is negative (over the positive range), the growth of the darker side is inhibited and the root curves towards this side.

The main biological process that consumes carbon dioxide is photosynthesis.

Sponges are different from other animals since they present only intracellular digestion. They do not have a digestive system nor do they release digestive enzymes in the spongocoel to cause extracellular break down of nutrients.

In mammals the embryonic development is direct, without larval stage.

The muscles of the legs, mainly the muscles of the calves, contract and compress the deep veins of the legs impelling the blood to the heart. The plantar portion of the feet retains blood and when it is compressed against the ground it impels its blood volume and aids venous return.

The cells that produce antibodies, i.e., the cells of the humoral immune system, are the B lymphocytes (B cells).

Thigmotropism is the movement or growth of the plant in response to mechanical stimuli (touch or physical contact), as when a plant grows around a supporting rod. It occurs for example in grape and passionfruit vines, etc.

The most abundant nitrogen-containing molecule found in nature is molecular nitrogen (N₂). The air is 80% constituted of molecular nitrogen. 

Reproduction in sponges can be asexual by budding, gemmation or fragmentation (regeneration) or sexual with larval stage (a ciliated amphiblastula larva).

Cetaceans (whales, dolphins) and sirenians (dugongs, manatees) are aquatic mammals. Chiropterans (bats) are flying mammals.

The arterial system has thicker muscle walls since within arteries the blood circulates under higher pressure. The veins are more flaccid than arteries. From the lumen to the external layer both types of vessels are made of endothelium, muscle tissue and connective tissue. In both the endothelium is a single layer of cells. In arteries the muscle tissue portion is thicker than in veins and in these vessels the external connective tissue is thicker than in arteries. Arteries are the pulsating blood vessels. The arterial pulse can be felt in a medical examination, for example, by the palpation of the radial artery in the internal and lateral face of the wrist near the base of the thumb.

Immunoglobulin is the alternate name given to antibody. Immunoglobulins are complex proteins containing an invariable portion and a variable portion and made of four polypeptide chains. The variable portion of each immunoglobulin is responsible for the high specificity of the antigen-antibody bond.

Fossil fuels, like oil, gas and coal, form when organic material is preserved from the complete action of decomposers, generally buried deep and under pressure over millions of years. Under such conditions the organic material transforms into hydrocarbon fuels.

Fossil fuels are a natural reservoir of carbon. When oxygen is present these fuels can be burned and carbon dioxide and carbon monoxide are released into the atmosphere. 

Apart from sponges, that do not have a digestive cavity where extracellular digestion takes place, all other animals have a digestive system with an internal cavity in which extracellular digestion occurs.

Bat and bird wings have the same function and the same origin (they are modified limbs) so they are analogous and homologous organs. Whale fins are a modification of the posterior limbs while fish fins although having the same function do not come from modified limbs; so they are analogous but not homologous structures.

Evolutionary homology suggests common ancestry while biological analogy relates to the concept of evolutionary convergence, the appearance of similar features in evolutionarily distant species that explore the same type of environment (in the mentioned case, the aquatic habitat).

The valves of the venous system are structures inside the veins that permit blood to flow only in the normal way (from the tissues to the heart) and forbid it to return in the reverse way in favor of gravity. The valves close when the pressure of the fluid column above (after, regarding normal flux) is higher than the fluid pressure before them. Valves are thus fundamental for the returning of blood to the heart.

The antibodies, or immunoglobulins, act to facilitate the destruction of antigens: they attract phagocytic leukocytes, they trigger the attack of specific defense molecules (activation of the complement system) and they directly neutralize the toxicity of some antigens.

Photoperiod is the daily time period of light exposure of a living being. The photoperiod may vary according to the period of the year.

Most living beings cannot use molecular nitrogen to obtain nitrogen atoms. Producers fix nitrogen mainly from nitrate (NO₃^-). Some plants also fix nitrogen from ammonia. Consumers and decomposers acquire nitrogen through digestion of mainly proteins and nucleic acids from the body of other living beings.

The sexual reproduction in sponges, in addition to contributing to genetic variability, also facilitates the colonization of farther environments by these beings, since sperm cells and larvae are mobile and can swim in the exterior to compensate the immobility of the adult individual.

Examples of representing beings: dogs, cats, horses, giraffes, elephants, apes, humans, bats, whales, dolphins, opossums, kangaroos, platypus. Basic morphology: hair, diaphragm muscle, mammary glands, enucleated red blood cells. Skin: impermeable, hairy.

Respiration: pulmonary. Circulation: closed and complete, heart with four chambers. Nitrogen waste: urea.

Thermal control: homeothermic. Types of reproduction: sexual, internal fecundation, oviparous monotremes (prototherians), marsupials (metatherians), placental (eutherians).

Varix means abnormal enlargement of veins. Varices occur when excessive pressure against the normal blood flux creates enlargement of the vein and thus insufficient functioning of its valves (venous insufficiency). Varices are more common in the veins of the inferior limbs since the fluid column above these vessels is higher. This is the reason why people that spend much time standing (e.g., surgeons) are more susceptible to varices. In general varices are not the apparent superficial veins that appear in the leg of varix patients. These  apparent vessels are the consequences of internal varices (venous insufficiency) in the deep internal veins of the legs and they appear because the blood flux is diverted to superficial veins. (Popularly however apparent superficial veins are called varices.)

Photoperiodism is the biological response presented by some living beings to their daily time of light exposure (photoperiod).

Leguminous crop rotation and other crop rotations are used in agriculture because in these plants many bacteria important for the nitrogen cycle live. The leguminous crop rotation (or cojointly with the main crop) helps the soil to become rich in nitrates that are then absorbed by the plants.

Green manure, the covering of the soil with grass and leguminous plants, is also a way to improve the fixation of nitrogen and it is an option in avoiding chemical fertilizers.

The gas exchange in sponges happens by diffusion from the exterior to the cells that absorb molecular oxygen and liberate carbon dioxide.

Atoms form molecules that form cells that form tissues that form organs that form systems. Systems, in their turn, constitute individuals that form populations that compound (biological) communities that form the biotic components of ecosystems. All ecosystems of earth form the biosphere.

The lymphatic system is a network of specialized valved vessels that drain interstitial fluid (lymph). The lymphatic system is also responsible for the transport of chylomicrons (vesicles that contain lipids) made after the absorption of fats by the intestinal epithelium. In the way of the lymphatic vessels there are ganglial-like structures called lymph nodes that contain many cells of the immune system. These cells filter impurities and destroy microorganisms and cellular wastes. The lymphatic vessels drain to two major lymphatic vessels, the thoracic duct and the right lymphatic duct, that in their turns drain into tributary veins of the superior vena cava.

Leaves are mainly responsible for perception of light intensity in plants. The pigment that performs this perception and commands photoperiodism is called phytochrome.

The nitrogen cycle represents the circulation and recycling of the chemical element nitrogen in nature. The nitrogen cycle basically depends on the action of some specialized bacteria. Bacteria of the soil called nitrogen-fixing bacteria present in plant roots absorb molecular nitrogen from the air and liberate nitrogen in the form of ammonia. The decomposition of organic material also produces ammonia. In the soil and roots (mainly of leguminous plants), a first group of chemosynthetic bacteria called nitrifying bacteria, the nitrosomonas, produces energy consuming ammonia and releasing nitrite (NO₂). The second group of nitrifying bacteria, the nitrobacteria, uses nitrite in chemosynthesis releasing nitrate (NO₃). In the form of nitrate, nitrogen is then incorporated by plants to be used as constituent of proteins and nucleic acids and the element then follows along the food chain. Nitrogen returns to the atmosphere by the action of denitrifying bacteria that use nitrogen-containing compounds from the soil and release nitrogen gas (molecular nitrogen). 

Some chemical substances secreted by sponges have anti-inflammatory, antibiotic and anti tumoral activities and they are used in the production of medicines. Since ancient times the endoskeleton of some sponges has had commercial value, they are used as cleansing implements for baths (bath sponges), to wash animals, objects and so on.

The lymph nodes, or lymph glands, have lymphoid tissue that produces lymphocytes (a type of leukocyte). In inflammatory and infectious conditions the enlargement of lymph nodes of the lymphatic circuits that drain the affected region due to the reactive proliferation of leukocytes is common. This enlargement is known as lymphadenomegaly and sometimes it is accompanied by pain. The search for enlarged or painful lymph nodes is part of the medical examination since these findings may suggest inflammation, infection or other diseases.

The antibodies of the humoral immune system act against extracellular agents, like toxins or bacteria, but they are not active in the intracellular space and they cannot fight virus efficiently.

In case of viral infection (and also of cancerous or precancerous cells) the immune attack is made by the cellular immune system, mediated by T and NK (natural killers) lymphocytes that destroy specific cells and virus.

Flowering is a typical and easy to observe example of photoperiodism. Most flowering plants flower only during specific periods of the year or when placed under some conditions of daily illumination. This occurs because their blossoming depends on the duration of the photoperiod that in its turn varies with the season of the year. Flowering is also affected by exposition to certain temperatures.

Biological diversity is the variety of species of living beings of an ecosystem. In ecosystems which are more biodiverse, like tropical forests, a great variety of plants, microorganisms and animals live; in ecosystems less biodiverse, like deserts, there are less variety of living beings.

Example of representing beings: sponges. Basic morphology: tubular or globular body with spongocoel, sessile; choanocytes, pinacocytes and amoebocytes. Type of symmetry: not established. Germ layers and coelom: do not apply since poriferans do not have true tissue organization. Digestive system: nonexistent. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: nonexistent. Types of reproduction: asexual and sexual with larval stage.

The heart chambers through which blood enters the heart are the atria. There are the right atrium and the left atrium. The heart chambers through which the blood exits the heart are the ventricles. There are the right ventricle and the left ventricle.

The lymphocytes that participate in the cellular immune response are the T lymphocytes. T lymphocytes differentiate into three main types: cytotoxic T lymphocytes (cytotoxic T cell), helper T lymphocytes (helper cell) and suppressor T lymphocytes. The cytotoxic cells are the effectors of the system, i.e., they directly attack other cells recognized as foreign (for example, fungi cells, cells infected by virus, neoplastic cells, graft cells, etc.). The helper cells and the suppressor T lymphocytes act as regulators of the system releasing substances that respectively stimulate and inhibit the immune action of T and B lymphocytes. After the primary immune response memory T lymphocytes also remain in the circulation to provide faster and more effective reaction in case of future infections.

The availability of abiotic factors like light, moisture, mineral salts, heat and carbon dioxide, more or less conditions the biodiversity of an ecosystem.

Photosynthesis depends on water and light, and plants also need mineral salts, carbon dioxide and adequate temperature for their cells to work. In environments where these factors are not restrictive the synthesis of organic material (by photosynthesis) is at a maximum, plants and algae can reproduce easier, the population of these beings increase, potential ecological niches multiply and new species emerge. The large mass of producers makes viable the appearing of a diversity of consumers of several orders. In environments with restrictive abiotic factors, like deserts, the producers exist in small numbers and less diversity, a feature that thus extends to consumers and conditions fewer ecological niches to be explored.

Example of representing beings: sponges. Basic morphology: tubular or globular body with spongocoel, sessile; choanocytes, pinacocytes and amoebocytes. Type of symmetry: not established. Germ layers and coelom: do not apply since poriferans do not have true tissue organization. Digestive system: nonexistent. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: nonexistent. Types of reproduction: asexual and sexual with larval stage.

The ventricle walls are thicker than the atrium walls since ventricles are structures responsible for the pumping of the blood to the lungs or tissues. The muscular work of the ventricles is harder and their muscle fibers develop more. The left ventricle is more muscular than the right ventricle because pumping  blood to the lungs (the right ventricle task) is easier (needs less pressure) than pumping blood to the other tissues of the body (the left ventricle task).

The antigen-presenting cells of the immune system, also known as APC cells, are cells that do phagocytosis and digestion of foreign (to the body) microorganisms and later expose antigens derived from these microorganisms in the outer side of their plasma membrane. These processed antigens are then recognized by lymphocytes that activate the immune response. Several types of cells, like the macrophages, can act as antigen-presenting cells. 

According to their photoperiodism-based flowering plants classify as long-day plants, those that depend on longer photoperiods than the critical photoperiod to flower, as short-day plants, those that depend on shorter photoperiods than the critical photoperiod to flower, and as indifferent plants, whose flowering does not depend on the photoperiod.

Jellyfish, hydra, corals and sea anemones are good examples. All of them are aquatic and most live in the sea.

The vena cava are either of two large veins that debouch into the right atrium. The superior vena cava drains all blood that comes from the head, the superior limbs, the neck and the superior portion of the trunk. The inferior vena cava carries blood drained from the inferior portion of the trunk and the inferior limbs. Venous blood circulates within the vena cava.

Active immunization is that in which an antigen penetrates the body triggering the primary immune response and the production of memory lymphocytes and antibodies that provide faster and more effective immune defense in future infections by the same antigen. Passive immunization is that in which immunoglobulins against an antigen are inoculated in the body to provide protection in case the body becomes infected by the antigen.

Active immunization tends to be longer lasting than passive immunization since in the active type as well as antibodies, specific memory lymphocytes remain in the circulation. In the passive immunization the duration of the protection is that of the duration of the antibodies in the circulation.

Phyllotaxis is the way leaves are arranged on shoots. Most plants have opposite phyllotaxis (alternating in sequence, one in one side of the shoot, the following in the opposite side) as a solution to prevent self shading of the leaves thus improving the efficiency of photosynthesis.

The natural soil of the Amazon Rainforest is not very fertile but it is enriched by the vegetal covering made of leaves and branches that fall from the trees. Deforestation reduces this enrichment. In deforestation zones the rain falls directly on the ground causing erosion, “washing” large areas (leaching) and contributing to make the soil even less fertile. Besides that, the deforestation disallows the recycling of essential nutrients for plants, like nitrogen. In this manner those regions and their neighboring regions undergo desertification.

Cnidarians have true tissue differentiation, they present distinct organized tissues in the body. Poriferans present only some dispersed specialized cells with no tissue differentiation.

The function of the right ventricle is to get venous blood from the right atrium and pump the blood to be oxygenated in the lungs. The venous blood is carried from the right ventricle to the lungs by the pulmonary artery and their ramifications.

Besides being nutritionally important, maternal milk participates in the defense of the baby against infectious agents. Soon after delivery the mother produces a more fluid milk called colostrum that is rich in immunoglobulins (antibodies). These antibodies are not absorbed by the baby’s circulation but they cover the internal surface of the baby’s bowels thus attacking possible antigens and making more difficult the proliferation of pathogenic bacteria within the organ.

A gene is a sequence of DNA nucleotides that codifies the production of a protein.

Monoculture means that in a large area a single crop (only one species of plant) is cultivated. Therefore monoculture does not contribute to the formation of a community with great variety of species in the area. Since there is only a single type of producer the types of consumers that can live in the area are also restricted.

These beings present ectoderm and endoderm, two germ layers. Animals with only two germ layers are called diploblastic animals.

The ectoderm gives birth to the epidermis and the endoderm originates the covering of the digestive cavity.

The venous blood that comes from the tissues arrives in the right atrium of the heart. From the right atrium the blood goes to the right ventricle. The valve that separates the right ventricle from the right atrium is the tricuspid valve (a valvular system made of three leaflets). The tricuspid valve is necessary to prevent returning of blood to the right atrium during systole (contraction of ventricles).

Antivenoms are obtained by the following process: the venom (antigen) is inoculated into other mammals, e.g., in horses; these animals make specific antibodies against the antigen; blood from the animals is collected and purified to get the antibodies; this antibody-containing material is the antivenom. When a human being is infected by the antigen the specific antivenom is given to him/her and the action against the antigen occurs.

Antivenoms may also be administered as a preventive measure and, since it is basically made of specific immunoglobulins against some antigen, the process is an example of passive immunization.

Diploid individuals have paired chromosomes. For example in humans there are 23 pairs of chromosomes totaling 46 chromosomes. Each pair comprehends homologous chromosomes, one chromosome from the father and another from the mother, both of them containing information related to the production of the same proteins (with the exception of the sex chromosomes, which are partially heterologous). So in the diploid individual it is said that each gene has two alleles, one in each chromosome of the homologous pair.

Very biodiverse areas present enormous economic potential. They can be a source of raw material for the research and production of medicines, cosmetics, chemical products and food. They are depositories of genetic wealth that can be explored by biotechnology. They are sources of species for agriculture. They can also be explored by 'ecological tourism'.

Morphologically, cnidarians classify as polyps or medusae. Polyps are cylindrical and medusae are circular convex, like an umbrella. Both shapes have tentacles.

In general polyps are sessile but some species, like hydra, can move by alternating contact points on the substrate and performing somersaults. Medusae can move expelling water jets by contraction of the body.

Some cnidarians alternate polypoid and medusoid forms in their life cycle.

The valve that separates the right ventricle and the base of the pulmonary artery is the pulmonary valve. The pulmonary valve is important to prevent blood from the pulmonary circulation to flow back to the heart during diastole.

A gene is not a triplet of DNA nucleotides with their respective nitrogen-containing bases, like AAG or CGT. The nucleotide triplets may be pieces of genes but not genes.

A gene is a portion of a DNA molecule that codifies a specific protein. Thus it is formed by several DNA nucleotide triplets.

In ecosystems with more biodiversity the food webs and ecological interactions among living beings are more complex and diverse. In these ecosystems environmental changes can be more easily compensated by the multiplicity of available resources, foods and survival options.

In ecosystems with less biodiversity the individuals are more dependent on some beings that serve them as food and they interact with a small number of different species. In these ecosystems generally abiotic factors are restrictive and the species are more specialized to such conditions and more sensitive to environmental changes. So even small environmental harm can cause big disturbances in the equilibrium of the ecosystem.

Incomplete digestive system is that in which the digestive cavity has only one opening.

Arteries of the pulmonary circulation are arteries that carry venous blood and not arterial blood. When the blood passes within the alveolar capillaries of the lungs hematosis (oxygenation) occurs and carbon dioxide is released to the exterior.

Natural active immunization is that in which a previous natural infection induces the primary immune response, specific memory cells are produced and the individual becomes immune to new infections with the antigen. This is what happens in diseases that affect people only once in life, like mumps and chickenpox.

Artificial active immunization is that in which the primary immune response is caused by the inoculation into an individual of specially prepared antigens. This is the case with vaccines.

A chromosome is a DNA molecule. A chromosome may contain several different genes and also DNA portions that are not genes.

The biggest dangers to biological diversity today are the action of humans. The main of them is the destruction of habitats caused by the growth of the cities, deforestation, pollution and fires. The second is the invasion of ecosystems by nonnative species introduced by humans; these species change the equilibrium of ecosystems causing harm. Other big dangers are predatory hunting and fishing and global warming. 

These animals have a digestive cavity and they make extracellular and intracellular digestion. The extracellular digestion takes place within the digestive cavity.

The function of the left ventricle is to get blood from the left atrium and to pump the blood under high pressure to the systemic circulation. After leaving the left ventricle the blood enters the aorta, the largest artery of the body.

The goal of vaccines is to artificially induce a specific primary immune response(and the consequent formation of and memory cells) concerning a given infection or disease in order to immunize the individual against infections by the pathogenic agent in the future.

Since each antibody does not act against a variety of antigens but instead it acts only against its specific antigen, it is necessary for the immune system to make contact in some way with the antigen against which the immunization is wanted. The reconnaissance of specific molecular portions of each antigen causes the immune system to produce the specific variable portion of the immunoglobulins to attack that antigen. Therefore to induce the active immunization it is necessary to inoculate into the body small parts of the infectious agent or the agent entirely (dead or inactivated).

Gene locus (locus means place) is the location of a gene in a chromosome, i.e., the position of the gene in a DNA molecule.

Harmonious, or positive, ecological interaction is that in which none of the participating beings is harmed.

Cnidocytes are specialized cells present in coelenterates. They are found in the epidermis and contain toxic substances that can hurt, paralyze or even kill other animals.

Each cnidocyte has an internal capsule know as nematocyst where the actual urticating substance is stored. When a cnidocyte is excited it causes the nematocyst to expose a filament containing the toxic chemical.

Cnidocytes and their nematocysts have the biological functions of defending the individual from external aggression and of helping to capture their prey.

The pulmonary veins are part of the pulmonary circulation. They are vessels that carry oxygen-rich (arterial) blood from the lungs to the heart. There are four pulmonary veins, two that drain blood from the right lung and other two that drain the left lung. The pulmonary veins debouch into the left atrium bringing arterial blood to the heart. Although they are veins they carry arterial blood and not venous blood.

Vaccines can be constituted of dead agents of disease, of inactivated agents of disease, of inactivated toxins or of fragments of the infectious agent. Examples of some vaccines and their type of antigenic agents are: BCG, inactivated tuberculosis bacilli; antitetanic vaccine, inactivated toxin; antidiphtheric, inactivated toxin; antipolio Salk, dead poliovirus; antipolio Sabin, attenuated (inactivated) poliovirus.

It is natural that alleles have come one from the father and the other from the mother but it is not obligatory. In a “clone” generated by nucleus transplantation technology, for example, the alleles come from a single individual. In polysomies (as in trisomy 21) each gene of the affected chromosome has three alleles, in trisomies, or four, in tetrasomies.

Inharmonious, or negative, ecological interaction is that in which at least one of the participating beings is harmed. 

Their nervous system is diffuse, there are no brain or ganglia.

The valve between the left ventricle and the aorta is the aortic valve. The aortic valve prevents the retrograde flux of blood to the left ventricle during diastole. Besides, as the aortic valve closes during diastole, part of the retrograde blood flux is impelled through the coronary ostia (openings), orifices located in the aorta wall just after the valvular insertion and contiguous to the coronary circulation responsible for the blood supply of the cardiac tissues.

Viruses that present a high mutation rate like the virus that causes the common cold escape easily from the action of vaccines against them. After a primary immune response (natural or artificially induced) against the virus in the next season of infection new mutant resistant strains appear and the protection obtained with the immune response of the last season is lost. (One could say that the high mutation rate is a form of “immunization” found by these viruses.)

A phenotype is every observable characteristic of a living being conditioned by its genes. Some phenotypes may be altered by nongenetic factors (for example, artificial hair coloring). Specific phenotypes are also called phenotypical traits.

The main harmonious intraspecific ecological interactions are colonies and societies. The main inharmonious intraspecific ecological interactions are intraspecific competition and cannibalism. 

They present asexual and sexual reproduction.

The arterial blood that has come from the lungs to the left atrium passes then to the left ventricle. The valve between the left ventricle and the left atrium is the mitral valve, a bicuspid (two leaflets) valve. The mitral valve is important because it prevents the regurgitation of blood to the left atrium during systole(contraction of the ventricles). 

Vaccines are not used in the treatment of infections because they depend on the primary immune response that takes about a week to occur and is not so intense and effective. Antivenom serums however are inoculated into the circulation and used as an immediate treatment because they are made of a great amount of immunoglobulin (antibodies) which is potent against their respective specific venom.

Genotype is the genes, DNA nucleotide sequences contained in the chromosomes of an individual, that condition the phenotype. Phenotypes then are a biological manifestation of genotypes.

For example, the altered hemoglobin chain of sickle cell disease and the manifestation of the disease itself are the phenotype. The altered DNA nucleotide sequence in the gene that codifies the production of that abnormal hemoglobin chain is the genotype.

Ecological interactions are classified as intraspecific or interspecific interactions and as harmonious or inharmonious interactions. 

Hydras reproduce asexually by budding.

Systole is the stage of the cardiac cycle on which the ventricles contract. So the lumen of these chambers is reduced and the pressure upon the blood within them is heightened. During diastole the opposite occurs. The  muscle fibers of the ventricles relax and the lumen of these chambers enlarges helping the entrance of blood.

The DNA vaccine, or DNA vaccination, is a vaccination technology based on genetic engineering. In DNA vaccination a recombinant plasmid (vector) containing the gene of a specific antigen that is part of a given pathogenic agent is inserted into cells of the individual to be immunized. These cells then begin to produce the antigen that triggers the primary immune response and theoretically the individual becomes immunized against that antigen.

A phenotype may be altered (compared to the original situation conditioned by its genotype) by nongenetic means. Examples: some hormones may cease to be secreted due to diseases but the genes that determine their secretion remain intact; a person can go to a hairdresser and change the color of his/her hair; plastic surgery can be performed to alter facial features of an individual; colored contact lenses may be worn; a plant can grow beyond its genetically conditioned size by application of phytohormones.

Revealing cases of environmental influence on phenotypes are observed in monozygotic twins that have grown in different places. Generally these twins present very distinct phenotypical features due to the environmental and cultural differences of the places where they lived and to their different individual experiences in life.

(Biologically programmed phenotypical changes, like nonpathological changes of the skin color caused by sunlight exposure, tanning, or the variation of the color of some flowers according to the pH of the soil cannot be considered independent from the genotype. Actually these changes are planned by the genotype as natural adaptations to environmental changes.)

Colonies are functional integrated aggregates formed by individuals of the same species. Colonies are often confused with a single individual. Examples are the coral reefs, by-the- wind sailors and filamentous algae.

Societies are interactions for labor division and collaboration among individuals of the same species. Human societies are examples of ecological societies; other species, like bees, ants, termites, wolves and dolphins, also form societies.

Metagenesis is the type of life cycle in which there are two different forms of individuals of the same species, one haploid and the other diploid. In one of these stages gametogenesis occurs and fecundated gametes give birth to the zygote that then develops into the other form. Metagenesis is also known as alternation of generations or as diplobiont life cycle. (All plants, for example, present metagenesis.)

The filling of the ventricles with blood occurs during diastole.

Diseases caused by the action of the own immune system of the individual are called autoimmune diseases. The autoimmune diseases appear when the immune system makes antibodies or defense cells that attack cells, tissues and organs of its own body. The attacked cells or tissues are wrongly recognized as antigens by the immune system. Rheumatoid arthritis, lupus, scleroderma, vitiligo, pemphigus, type I diabetes mellitus, Crohn's disease (chronic inflammation of the gut), myasthenia gravis, Graves disease, Hashimoto's disease, etc., are all examples of autoimmune diseases.

Changes caused on phenotypes by the environment are not transmitted to the offspring (unless their primary cause is genotypical change in germ cells or in gametic cells). If a person changes the color of the hair or undergoes aesthetic plastic surgery the resulting features are not transmitted to his/her offspring.

Intraspecific ecological interactions are those between individuals of the same species. Interspecific ecological interactions are ecological interactions between individuals of different species. 

In the metagenesis of some coelenterates, like Aurelia and Obelia, there is alternation between polypoid and medusoid forms. The polyps reproduce asexually originating medusae that then liberate gametes. The male and female gametes unite under water to form the zygote that then develops into larva and later originates another polyp.

The form that reproduces asexually is the polyp. Polyps give birth to medusae by budding in Obelia and by strobilization in Aurelia.

Respiratory pigments are oxygen- carrying molecules present in the blood. When the oxygen concentration is high, for example, in the pulmonary alveoli, the respiratory pigments bind to the gas. In conditions of low oxygen concentration, e.g., in tissues, the respiratory pigments release the molecule. In the human blood the respiratory pigment is hemoglobin, present within the red blood cells.

Gametes are cells specialized in sexual reproduction. They contain half of the maximum number of chromosomes of the species and unite with another gamete giving birth to a zygote with double of the number of chromosomes of the gametic cells. In humans gametes are formed by meiosis; the male gametes are the sperm cells and the female gametes are the egg cells.

The environment can only alter genotypes when its action causes alterations in the genetic material (mutations) of the individual, i.e., deletion, addition or substitution of entire chromosomes or of nucleotides that form the DNA molecules.

Mutations are only transmitted to the offspring when affecting the germ cells that produce gametes or the gametes themselves.

Inharmonious, or negative, ecological interaction is that in which at least one of the participating beings is harmed. 

Sexual reproduction in corals and sea anemones have a larval stage and the larva is called planula.

Many marine animals are sessile or practically sessile, like sponges, corals and sea anemones. The mobile larval stage of their life cycle provides better spatial distribution of these species.

The heart is made of striated cardiac muscle tissue. The heart muscle is called the myocardium and it is oxygenated and nutrified by the coronary arteries. The coronary arteries come from the base of the aorta and ramify around the heart penetrating the myocardium. Diseases of the coronary arteries are severe conditions. 

The cells that form gametes are the germ cells as opposed to the somatic cells. The ploidy (number of chromosomes) of the germ cells is the same as the somatic cells (only during the formation of gametes meiosis occurs and the number of chromosomes is reduced to half).

Competition is the ecological interaction in which the individuals explore the same ecological niche or their ecological niches partially coincide and therefore competition for the same environmental resources takes place.

Competition is harmful for all participating beings and thus it is classified as an inharmonious (negative) ecological interaction. 

Coelenterates are divided into three main classes: hydrozoans, scyphozoans and anthozoans. In hydrozoans the polypoid form predominates and examples are hydras, by-the-wind- sailors and Obelia. In scyphozoans the main phase is the medusoid and the best known example is the common jellyfish (Aurelia). In anthozoans there is only the polypoid form and corals and sea anemones are notable in this group.

Oxygen-bound hemoglobin is called oxyhemoglobin. In the lungs the oxygen concentration is higher and so there is a higher oxyhemoglobin concentration. In the peripheral tissues the situation is the reverse, the concentration of oxygen is lower and there is more free hemoglobin.

Gonads are the organs that produce gametes. They contain the germ cells that undergo division and generate gametes. In males the gonads are the testicles. In females the gonads are the ovaries.

If a gene of a diploid species has different alleles, for example, A and A’, the possible genotypes are: A’A’, AA, and AA’. So any of these three different genotypes may be the genotype of an individual.

The main harmonious interspecific ecological interactions are: protocooperation, mutualism and commensalism. The main inharmonious interspecific ecological interactions are: interspecific competition, parasitism, predatism and ammensalism. 

Radial symmetry means (biologically) that the animal structures are situated in a radial or circular pattern around a center point with nonexistence of sides, like right or left. An alternative type of symmetry in which structures are placed equally in the sides of a longitudinal axis is the lateral symmetry (the symmetry present in human beings, for example).

Chordates present lateral symmetry.

Besides cnidarians another animal phylum with species presenting secondary radial symmetry is the phylum Echinodermata. (It is considered that the simplicity of poriferans does not characterize any symmetry.)

The main metabolic gases transported by the blood are molecular oxygen (O₂) and carbon dioxide (CO₂).

Meiosis is the type of cell division that allows sexual reproduction since it reduces to a half the number of chromosomes of the species making possible the combination of two gametes to form a new individual. (In some beings meiosis creates haploid gametophytes that by means of mitosis generate gametes. Even in this case the function of meiosis is the same: to provide cells with half of the number of chromosomes of the species with separation of the homologous.) Gametogenesis is the name given to the process of gamete production. 

Protocooperation is the ecological interaction in which both participants benefit but which is not obligatory for their survival. Protocooperation is a harmonious (positive) interspecific ecological interaction. Examples of protocooperation are: the action of the spur-winged plover that using its beak eats residuals from crocodile teeth; the removal of ectoparasites from the back of bovines by some birds that eat the parasites; the hermit crab that live inside shells over which sea anemones live (these offer protection to the crab and gain mobility to obtain food). 

Corals are characterized by their polypoid shape, sessility and slow growth and secretion of a solid skeleton  made of calcium carbonate. Numerous coral individuals associate in intraspecific harmonic ecological interaction forming colonies with hundreds, thousands and even millions of beings. Water-permeated solid structures of these colonies, known as reefs, work as true ecosystems for other living beings. The biggest known coral colony is the Great Barrier Reef in the northeast coast of Australia. There are however many coral species whose individuals live alone and do not form colonies.

Hemoglobin F is the hemoglobin found in the mammalian fetus and hemoglobin A is the normal hemoglobin.Hemoglobin F has higher affinity for oxygen. The fetus needs hemoglobin capable of extracting oxygen from the mother’s circulation. Therefore the fetus uses hemoglobin F since it has higher affinity for oxygen than the mother’s hemoglobin.

The formation of sperm cells, or spermatogenesis, begins with a germ cell called spermatogonium (2n) that suffers mitosis and gives birth to the spermatocyte I (2n). The spermatocyte I undergoes meiosis I and generates two spermatocyte II (n) that then undergo meiosis II and produce four spermatids (n). Each spermatid undergoes a maturation process called spermiogenesis and four sperm cells appear.

Not in all cases of a gene having two different alleles is the dominance complete. There are genes in which heterozygosity occurs with incomplete dominance (manifestation of an intermediate phenotype in relation to the homozygous, like in the color of roses, between white and red) and other genes that present codominance (expression of a third different feature, as in the MN blood group system).

Intraspecific competition occurs in practically all species, for example, the competition of humans for a job. 

Examples of representing beings: jellyfish, corals, sea anemones, hydra. Basic morphology: polyp or medusa.

Type of symmetry: radial. Germ layers and coelom: diploblastics, acoelomate. Digestive system: incomplete. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: diffuse. Types of reproduction: asexual and sexual with larval stage and metagenesis.

The substance that stimulates the production of red blood cells by the bone marrow is erythropoietin. Erythropoietin is a hormone secreted by the kidneys. Its secretion is increased when there is deficient tissue oxygenation (tissue hypoxia) caused either by reduced oxygen availability (as it occurs in high altitudes) or by internal diseases, as in pulmonary diseases.

Sperm cells (the male gametes) are matured spermatids that have already undergone differentiation (appearance of the flagellum, reduction of the cytoplasm, formation of the acrosome, increase in the number of mitochondria). This differentiation process is called spermiogenesis.

Homozygosity occurs when an individual has two identical alleles of a gene, for example, AA or aa. Heterozygosity occurs when an individual has two different alleles of the same gene, in the example, Aa.

In cannibalism an individual eats other of the same species (occurs in some insects and arachnids). Since it is an interaction between beings of the same species and at least one of them is harmed (the other is benefited) the classification as inharmonious intraspecific ecological interaction is justified. 

The most popular representatives of the platyhelminthes are worms that cause human diseases, like taenia and schistosome. The planaria, since it is been extensively studied in Biology, is also well known.

In high altitudes the air is rarefied and oxygen concentration is lower than in low altitudes. In this situation the efficiency of the respiratory system must be greater and thus the organism synthesizes more hemoglobin (and more red blood cells) trying to get more oxygen. This phenomenon is known as compensatory hyperglobulinemia. The compensatory hyperglobulinemia is the reason why athletes that will compete in high altitudes need to arrive in the place some days before the event so there is time for their body to make more red blood cells and they will be less affected by the effects of the low atmospheric oxygen concentration (fatigue, reduced muscular strength).

The male germ cells are the spermatogonia (diploid cells, 2n) situated in the testicles. They mature and by means of mitosis give birth to spermatocytes I (2n) that will undergo meiosis.

Dominant allele is the allele that determines phenotypical features that manifest in homozygous or heterozygous genotypes. In Genetics the dominant allele is represented in uppercase, e.g., “A”, and its recessive allele is written in lowercase, “a”. In molecular terms generally the recessive allele has a nucleotide sequence previously identical to the corresponding sequence in the dominant allele but that during evolution was inactivated by mutation. This fact explains the expression of the dominant phenotype in heterozygosity (since one functional allele is still present).

Mutualism is the ecological interaction in which both participants benefit and that is obligatory for their survival. Mutualism is a harmonious (positive) ecological interaction. Mutualism is also known as symbiosis. Examples of mutualism are: the association between microorganisms that digest cellulose and the ruminants or insects within which they live; the lichens, formed by algae or cyanobacteria that make organic material for the fungi and absorb water with their help; nitrifying bacteria of the genus Rhizobium that associated to leguminous plants offer nitrogen to these plants.

Flatworms have incomplete digestive systems and they present extracellular and intracellular complementary digestions.

Hemoglobin “likes” carbon monoxide (CO) much more than it likes oxygen. When there is carbon monoxide in the inhaled air it binds to hemoglobin forming carboxyhemoglobin by occupying the binding site where oxygen would bind. Due to the higher hemoglobin affinity for carbon monoxide thus (e.g., in intoxication from car exhausts) there is no oxygen transport and the individual undergoes hypoxia, loses conscience, inhales more carbon monoxide and may even die. Intoxication by carbon monoxide is an important cause of death in fires and in closed garages.

The spermatocyte I (2n) undergoes the first division of meiosis (meiosis I) originating two spermatocyte II (haploid, n).

A recessive allele can remain hidden because it does not manifest in heterozygous individual, i.e., it may be present in the genotype but not expressed in the phenotype. When this allele is transmitted to the offspring and forms homozygous genotype with another recessive allele from other chromosomal lineage the phenotypical characteristics that come out reveal its existence.

Commensalism is the ecological interaction in which one individual benefits while the other is neither benefited nor harmed. Commensalism is a harmonious (positive) ecological interaction, since none of the participants is harmed. An example of commensalism is the numerous bacteria that live in the skin and in the digestive tube of humans without being pathogenic or beneficial. They are innocuous bacteria living in commensalism with humans. 

Planarias have single opening digestive system (incomplete) with ramifications that transport nutrients to all areas of the body.

In aerobic cellular respiration the release of carbon dioxide happens in the transformation of pyruvic acid into acetyl-CoA (two molecules) and in the Krebs cycle (four molecules). For each glucose molecule, six carbon dioxide molecules are made.

The acrosome is a structure that contains a great number of digestive enzymes, it is located in the anterior end of the sperm cell and it is formed by the union of Golgi apparatus vesicles. The function of the acrosome is to release its enzymes when the sperm cell meets the egg cell to break the external covering of the female gamete thus making fecundation possible.

Mendel is considered the father of Genetics. He was a monk, biologist and botanist born in Austria in 1822 and who died in 1884. During the years 1853 to 1863 he cultivated pea plants in the gardens of his monastery to be used in his research. His experiments consisted of crossing pea plants of distinct characteristics (size, color of the seeds, etc.), cataloging the results and interpreting them. The experiments led him to enunciate his laws, results published in 1886 with no scientific repercussion at that time. Only at the beginning of the 20th century, in 1902, 18 years after his death, were his merits broadly recognized.

Parasitism is the ecological interaction in which a being lives at the expense of another. The parasite often does not cause immediate death of the host since it needs the host alive to survive. Parasitism is an inharmonious (negative) interspecific ecological interaction, since although one participant benefit the other is harmed. 

Platyhelminthes are also known as flatworms because they are worms with a flat body. This is the main external morphological feature that differentiates them from nematodes (roundworms).

The fish heart is a tube made of two consecutive chambers: one atrium and one ventricle.

The spermatids (n) are the products of the second division of meiosis (meiosis II) in the male gametogenesis. Each spermatocyte II originates two spermatids totaling four spermatids for each spermatocyte I that enter meiosis.

Monohybridism is the study of only one characteristic in the crossing of two pure individuals (hybridization) for that characteristic.

Classical examples are the parasites of humans (host), like the trypanosome that causes Chagas’ disease, the HIV virus (AIDS), the bacteria that causes tuberculosis, the schistosome that causes schistosomiasis, the hookworms, etc. Other examples are: tree (host) and parasitic helminths (parasite), dog (host) and lice (parasite), cattle (host) and tick (parasite), etc. 

Platyhelminthes exchange gases exclusively by diffusion through their body surface. This is only possible because all cells are localized relatively near to the exterior since gases diffuse cell by cell (the flat shape of these worms is a feature that allows this type of respiration).

In vertebrates almost 70% of the carbon dioxide is transported by the blood in the form of bicarbonate, 25% bound to hemoglobin and 5% dissolved in the plasma.

The flagellum of the sperm cell is made by the centrioles that migrate to the region posterior to the nucleus. Its function is to promote locomotion towards the egg cell.

Hybridization in Genetics is the crossing of individuals from “pure” and different lineages in relation to a given trait, i.e., the crossing of different homozygous for the studied trait.

In Mendel’s experiments with peas, for example, a plant from a green pea lineage obtained from self fecundation of its ascendants through several generations was crossed (cross fecundation) with another plant from a yellow lineage also obtained by self fecundation of ascendants. (The self fecundation through several generations of ascendants and the exclusive obtainment of individuals with the desired characteristics ensured that the individuals of the parental generation were “pure”, i.e., homozygous for that characteristic.)

Examples of interspecific competition are: the dispute among vultures, worms, flies and microorganisms for carrion and the competition between snakes and eagles for rodents. 

Platyhelminthes are the first triploblastic animals (remember that cnidarians are diploblastic), i.e., they present three germ layers: ectoderm, mesoderm and endoderm.

The venous blood coming from the tissues enters the atrium and passes to the ventricle that then pumps the blood towards the gills. After oxygenation in the gills the arterial blood goes to the tissues. So the fish heart pumps venous blood.

The reduced cytoplasm of sperm cells decreases the cell weight and provides a more hydrodynamic shape for the locomotion in fluids. The high concentration of mitochondria at the base of the flagellum of the sperm cell is necessary for the energetic supply of the flagellum (for it to beat and move the sperm cell).

In relation to genotypes and phenotypes the hybridization comprises of: parental generation (P): RR (read), yy (yellow). F1 generation (RR x yy): Ry (red). F2 generation (Ry x Ry): RR (red), Ry (red), Ry (red) and yy (yellow).

In the F1 generation the proportion of red flowers is 100%. In the F2 generation, the phenotypical proportion is three red (75%) to one yellow (25%).

Predatism is the ecological interaction in which one individual mutilates or kills another to get food. Predatism is an inharmonious (negative) ecological interaction since one participant is harmed.

Platyhelminthes have a primitive excretory system made of flame cells (also called solenocytes), excretory ducts and excretory pores.

Double closed circulation, or closed circulation, is that in which the blood circulates through two associated and parallel vascular systems: one that carries blood to and takes blood from the peripheral tissues (the systemic circulation) and the other that carries blood to and takes blood from the tissues that perform gas exchange with the environment, e.g, the lungs (pulmonary circulation). Double circulation occurs in  amphibians, reptiles, birds and mammals. Simple closed circulation, or simple circulation, is the one in which the tissues that perform gas exchange are associated in series with the systemic circulation, as in fishes.

The formation of egg cells begins with a germ cell called oogonium (2n) that undergoes mitosis and gives birth to the oocyte I (2n). The oocyte I undergoes meiosis I that however is interrupted at prophase. After puberty during each menstrual cycle an oocyte I finishes the meiosis I and generate one oocyte II (n) and the first polar body (n). With fecundation the oocyte II then undergoes meiosis II and produces the mature egg cell (n) and the second polar body (n).

In this monohybridism one of the colors does not appear in the F1 generation because their parental generators are pure, i.e., homozygous, and in F1 all descendants are heterozygous (each parental individual forms only one type of gamete). Since only heterozygous genotypes appear and red is dominant over yellow the individuals of the F1 generation will present only red flowers. 

Herbivorism is a form of predatism in which first order consumers feed from producers (plants or algae). For example, birds and fruits, humans and edible vegetables, etc. (There are proposals to consider the herbivorism of leaves a form of parasitism and the herbivorism of entire plants and seeds a form of predatism). 

Platyhelminthes of freshwater, like planarias, have an internal environment much more concentrated in solutes than the exterior and so they present a tendency to gain water. These organisms then need a drainage system to avoid cell death caused by excessive water.

The problem is solved by the presence of protonephridia located along longitudinal channels in the animal body. Protonephridia have ciliated cells, the flame cells, that push water outside the body through excretory pores.

Complete circulation is that in which there is no mixture of venous blood and arterial blood. Simple circulation is that in which the blood circulates only in one circuit (as opposed to the double circulation that have two circuits, the systemic circulation and the pulmonary circulation). In fishes the circulatory system is simple and complete.

The formation of spermatogonia in men takes place during the embryonic period. The formation of sperm cells however is a continuous process that begins in puberty and goes on until old age and sometimes during all the remaining life of the man. In women all oogonia are formed before birth. The oogonia turn into oocytes I that enter the first division of meiosis (meiosis I). This division however is interrupted at prophase and continues only in puberty. After the beginning of menses an egg cell is released during each period and, if fecundated, it finishes the meiotic division. The oogenesis stops after menopause (cessation of the menstrual activity) and the climacteric period of life begins.

To say that gametes are pure means that they always carry only one allele of the referred trait. Gametes are always “pure” because in them the chromosomes are not homologous, they contain only one chromosome of each type.

The amphibian heart has three heart chambers: two atria and one ventricle.

In oogenesis the oogonium differentiates into oocyte I (2n) and this cell enters meiosis. After finishing the first meiotic division (meiosis I) the oocyte I forms two cells: the oocyte II (n) and the first polar body. The oocyte II is bigger because it gets almost all the cytoplasm and the cytoplasmic structures of the oocyte I as a strategy for metabolite and nutrient storage. The oocyte II cell goes then to the second meiotic division. The first polar body is very small and almost lacks cytoplasm; it disintegrates or stays attached to the oocyte II.

In the monohybridism conditioned by two different alleles the F1 generation presents only heterozygous individuals (Bw). In F2 there is one individual BB, two individuals Bw and one individual ww. In relation to the phenotype there are in F2 two black individuals and one white individual, since black is the dominant color. So the proportion is 3:1, three black-haired to one white- haired.

Primary ecological succession is the changing sequence of communities from the first biological occupation of a place where previously there were no living beings. For example, the colonization and the following succession of communities on a bare rock.

Secondary ecological succession is the changing sequence of communities from the substitution of a community by a new one in a given place. For example, the ecological succession of the invasion of plants and animals in an abandoned crop or land.

Cephalization is the evolutionary tendency of concentration of the nervous command in central structures in which there are grouping of neurons (i.e, brain and ganglia formation).

Evolutionarily the cephalization process begins with the appearance of ganglia (group of neurons) in platyhelminthes and reaches an apex in vertebrates, animals with a cranial box to protect the well-developed brain.

With lateral symmetry the body can be divided into lateral portions, superior, inferior, anterior and posterior. These portions must be integrated and controlled in some manner and this need stimulated the appearance of ganglial complexity and of beings with a head, a privileged extremity of the bilateral body where the nervous central command and important sensory organs are located.

The amphibian circulation is double because it is composed of systemic and pulmonary circulations: respectively, heart-tissues-heart and heart-lungs- heart. Since amphibians have only one ventricle in the heart, venous blood taken from the tissues and arterial blood coming from the lungs are mixed in the ventricle that then pumps the mixture back to the systemic and to the pulmonary circulations. The amphibian circulation is classified as incomplete because venous and arterial blood mix in the circuit.

The blood oxygenation in amphibians occurs also in the systemic circulation since their skin is a gas exchange organ.

The oocyte II only completes the second meiotic division (interrupted at metaphase) if fecundation by a male gamete occurs. (One can say therefore that in fact the female gamete is the oocyte II).

The Mendel’s first law postulates that a characteristic (trait) of an individual is always determined by two factors, one inherited from the father and the other from the mother and the direct offspring of the individual receives from it only one of these factors (aleatory). In other words, each trait is determined by two factors that segregate during gamete formation. 

The Mendel’s first law is also known as the law of purity of gametes. Mendel deduced the way genes and alleles were transmitted and traits were conditioned without even knowing of the existence of these elements.

Ecological succession is the changing sequence of communities that live in an ecosystem during a given time period. 

Platyhelminthes may present sexual or asexual reproduction.

The reptiles have double and incomplete circulation too, three heart chambers (two atria and one ventricle). The reptile heart however presents the beginning of a ventricular septation that partially separates a right and left region of the chamber. With the partial ventricular septation the mixture of arterial with venous blood in the reptile heart is less than in amphibians.

The sperm cell that reaches the egg cell triggers the acrosome reaction, a process in which hydrolytic enzymes of the acrosome are released on the external surface of the zona pellucida (the protective layer that surrounds the egg cell). A portion of this layer is digested by the acrosomal enzymes allowing the sperm cell to reach the plasma membrane of the egg cell carrying out fecundation. At the moment that the sperm cell makes contact with the egg cell membrane a chemical alteration of this membrane occurs. Enzymes secreted by exocytosis (cortical reaction) make the zona pellucida unable to bind to other sperm cells (zonal reaction) and other male gametes cannot enter the egg cell.

In the mentioned hybridization the genotypical forms in F2 will be TT, tt and Tt. Therefore there will be three different genotypical forms and two different phenotypical forms (considering T dominant over t).

Pioneer species are those first species that colonize places where previously there were no living beings, like, for example, algae that colonize bare rocks. In general, pioneer species are autotrophs or those that maintain harmonious ecological interaction with autotrophic beings (like autotrophic bacteria, herbaceous plants, lichens).

The pioneer community is formed of species able to survive under hostile environments. The presence of these species modifies the microenvironment generating changes in abiotic and biotic factors of the ecosystem undergoing formation. Therefore they open the way to other species to establish in the place by the creation of new potential ecological niches.

Planarias can divide themselves asexually by transversal bipartition due to the great regeneration capability of their tissues. When they attach to a substrate they can induce a constriction in their middle region and the body is then separated into two parts and each of these parts gives birth to a new individual as tissue regenerates.

The bird and the mammalian hearts are divided into four chambers: right atrium, right ventricle, left atrium and left ventricle. Birds and mammals are homeothermic, i.e., they control their body temperature. The four-chambered heart and the double circulation provide the supply of more oxygenated blood to the tissues making possible a higher metabolic rate (mainly cellular respiration rate). Part of the energy produced by the cellular respiration is used to maintain the body temperature.

After termination of the second meiotic division of the oocyte II two cells are generated: the egg cell proper and the second polar body. The second polar body is a very small cell that almost lacks cytoplasm and stays adnexal to the egg cell. The entire cytoplasmic content of the oocyte II passes to the egg cell.

If an individual is dominant homozygous, for example, AA, it will produce only gametes having the allele A The proportion thus is 100% of AA gametes. If an individual is recessive homozygous, for example, aa, it will produce only gametes having the allele a, also in a 100% proportion.

The climax stage is the stage of the ecological succession in which the community of an ecosystem becomes stable and does not undergo significant changes. In the climax community practically all ecological niches are explored and greater biodiversity is possible. In this stage the biomass, the photosynthesis rate and the cellular respiration reach their maximum levels and thus the net primary production (NPP = organic material made by the producers – organic material consumed in the cellular respiration of the producers) tends to zero. At the climax the amount of oxygen released by photosynthesis is practically equal to the oxygen consumed by respiration. (This is one more reason why it is wrong to say that the Amazon Rainforest, an ecosystem at climax stage, is “the lung” of the earth. Other reasons are: lungs are not producers of oxygen; the algae and cyanobacteria of the phytoplankton are the main producers of the molecular oxygen of the planet.) 

There are monoecious hermaphrodite flatworms, like planarias and taenias, and there are dioecious (having male and female individuals) species too, like schistosomes. 

The reptiles have double and incomplete circulation too, three heart chambers (two atria and one ventricle). The reptile heart however presents the beginning of a ventricular septation that partially separates a right and left region of the chamber. With the partial ventricular septation the mixture of arterial with venous blood in the reptile heart is less than in amphibians.

Ovulation is the releasing of the female gamete from the ovary. Ovulation is a periodical event that occurs during each menstrual cycle. Considering as the first day of the menstrual cycle the day when menses begins, the ovulation occurs around the 14th day when the concentrations of the hormones LH and FSH reach high levels.

Heterozygous individuals, for example, AA, produce two different types of gametes: one containing the allele A and another type containing the allele a. The proportion is 1:1.

Biodiversity, the number of living beings and the biomass of an ecosystem tend to increase as the succession progresses and they stabilize when the climax stage is reached.

At the initial stage of the succession the use of carbon dioxide and the fixation of carbon into the biomass are high, since the total number of living beings in the ecosystem is increasing. At the climax stage the use of carbon dioxide by photosynthesis equals the production by cellular respiration and the fixation of carbon into the biomass tends to zero.

There are hermaphrodite species of animals and plants that present cross- fecundation mainly due to the maturation of female and male structures at different periods.

Cross fecundation occurs in planarias, hermaphrodites in which sexual fecundation takes place with male and female gametes from different individuals. These individuals approach their copulating structures and exchange gametes.

In the human fetal circulation there are two communications between arterial and venous blood characterizing an incomplete circulation. One of them is the oval foramen, an opening between the right and the left atria of the fetal heart. The other is the arterial duct, a short vessel connecting the pulmonary artery to the aorta. These communications close a few days after birth and so they are not present in the adult heart.

The female pronucleus is the proper haploid nucleus of the egg cell. Male pronucleus is the haploid nucleus of the sperm cell that has fecundated the egg cell. After fecundation both pronuclei fuse forming the nucleus of the diploid zygote.

From the crossing of an individual having recessive phenotype with another having dominant phenotype (for the same trait) it is possible to determine whether the dominant individual is homozygous or heterozygous. This is true because the genotype of the recessive individual is obligatorily homozygous, for example, aa. If the other individual is also homozygous, AA, the F1 offspring will be only heterozygous (aa x AA = only Aa). If the other individual is heterozygous there will be two different genotypical forms, Aa and aa in the 1:1 proportion. So if a recessive phenotype appears in the direct offspring the parental individual that manifests dominant phenotype is certainly heterozygous.

Migration is the moving of individuals of a species from one place to another. Emigration is the migration seen as an exit of individuals from one region (to another where they will settle permanently or temporarily). Immigration is the migration seen as the settling in one region (permanently or temporarily) of individuals coming from another region. Therefore individuals emigrate "from" and immigrate "to". 

Sexual reproduction with direct development is that in which there is not a larval stage in the embryonic development. When a larval stage exists it is said to be indirect development.

In the sexual reproduction of planarias there is no larval stage.

The female gametes are big cells full of vitellus (nutritive material). The male gametes are small, mobile and agile flagellate cells. Those features are related to their respective biological functions. While the female gametes have the basic functions of receiving the sperm cell nucleus and of storing nutrients for the zygote, the male gametes have the function of active movement towards the egg cell.

Step 1: to determine whether the studied phenotypical form has a dominant or recessive pattern. 

Step 2: to identify recessive homozygous individuals.

Step 3: to identify the remaining genotypes.

Population density is the relation between the number of individuals of a population and the area or volume they occupy. For example, in 2001 the human population density of the United States (according to the World Bank) was 29.71 inhabitants per square kilometer and China had a population density of 135.41 humans per square kilometer. 

Platyhelminthes are divided into three classes: turbellarians (or Turbellaria), trematodes (or Trematoda) and cestodes (or Cestoda).

Turbellarians are free-living platyhelminthes and their main representative is the planaria (Dugesia tigrina). Trematodes are parasites, they live inside a host and the schistosome (Schistosoma mansoni) that causes schistosomiasis is an example. Cestodes are parasites too, they do not have digestive tubes and their cells are nourished by absorption of nutrients from the host; their most popular representative are the beef and pork taenias (Taenia saginata and Taenia solium) that parasite humans.

Excretion in Physiology is the process of elimination of metabolic wastes and other toxic substances from the body.

The organs that comprise the male genital system are the testicles, the epididymides, the vas deferens, the seminal vesicles, the ejaculatory duct, the prostate, the bulbourethral glands, the urethra and the penis.

Genetic family tree is a schematic family tree that shows the biological inheritance of some trait through successive generations.

Genetic family trees are useful because it is practically impossible and ethically unacceptable to make experimental crossings for genetic testing between human beings. With the help of family tress the study is made by analysis of marriages (and crossings) that have already occurred in the past. From the analysis of family trees, for example, information on probabilities of the emergence of some phenotype and genotypes (including genetic diseases) in the offspring of a couple can be obtained.

The main factors that make populations grow are births and immigration. The main factors that make  populations decrease are deaths and emigration.  

The main human diseases caused by platyhelminthes are schistosomiasis, tapeworm disease (cestodiasis) and cysticercosis.

(Note:Diseases are studied in the “Diseases” division of this e-book.)

Nitrogen wastes are residuals derived from the degradation of proteins. They are made from chemical transformation of the amine group of amino acid molecules.

These secretions along with sperm cells from the testicles form the semen. The secretions have the function of nourishing the sperm cells and serving them as a fluid means of propagation. The basic pH of the seminal fluid also neutralizes the acid secretions of the vagina allowing the survival of sperm cells in the vaginal environment after copulation.

Mendel’s second law postulates that two or more different traits are also conditioned by two or more pair of different factors and that each inherited pair separates independently from the others. In other words, gametes are formed always with an aleatory representative of each pair of the factors that determine phenotypical characteristics.

Mendel’s second law is also known as the law of independent segregation of factors, or law of independent assortment.

Population growth rate (PGR) is the percent variation between the number of individuals in a population at two different times. Therefore the population growth rate can be positive or negative. 

Examples of representing beings: planarias, schistosomes, taenias. Basic morphology: flat worm. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, acoelomates.

Digestive system: incomplete. Respiratory system: nonexistent, respiration by diffusion. Circulatory system: nonexistent. Excretory system: protonephridia with flame cells. Nervous system: ganglial, beginning of cephalization. Types of reproduction: asexual and sexual.

The main nitrogen wastes excreted by living beings are ammonia, uric acid and urea. Living beings that secrete ammonia are known as ammoniotelic. Creatures that secrete uric acid are known as uricotelic. Organisms that secrete urea are called ureotelic.

The testicles are the male gonads, i.e., the organs where the production of gametes takes place. In human beings the gametes are made by meiosis that occur in the testicles.

In genetic family trees the male sex is usually represented by a square and the female by a circle. Crossings are indicated by horizontal lines that connect squares to circles and their direct offspring are listed below and  connected to that line. The presence of the studied phenotypical form is indicated by a complete hachure (shading) of the circle or the square correspondent to the affected individual. It is useful to enumerate the individuals from left to right and from top to bottom for easy reference.

Examples of migratory animals are: southern right whales from Antarctica, that procreate on the Brazilian coast; migratory salmons that are born in the river, go to the sea and return to the river to reproduce and die; migratory birds from cold regions that spend the winter in tropical regions, etc. 

Ascaris, hookworm and filaria, all parasites of humans, are examples of nematodes (also known as  roundworms).

Ammonia is a highly toxic molecule if not diluted and quickly excreted out of the body. For this reason the ammoniotelic excretion was abandoned in terrestrial habitats because the availability of water for dilution is reduced in this medium and wastes cannot be excreted so promptly to the exterior.

After leaving the epididymis in the testicle sperm cells enter the vas deferens, after that they receive secretions from the seminal vesicles and gather (from right and left sides) in the ejaculatory duct that passes inside the prostate. They also get secretions from the prostate and the bulbourethral glands and then go through the urethra, inside the penis, to the exterior.

Mendel’s second law is only valid for genes located in different chromosomes. For genes situated in the same chromosome, i.e., linked genes (genes in linkage) the law is not valid since the segregation of these genes is not independent.

Biotic potential is the capability of growth of a given population under hypothetical optimum conditions, i.e., in an environment without limiting factors to such growth. Under such conditions the population tends to grow indefinitely. 

There are parasitic roundworms, including parasites of plants, but there are also free-living nematodes.

Aquatic animals, like crustaceans, bony fishes and amphibian larvae, generally are ammoniotelic since ammonia diffuses more easily through membranes and it is more water soluble than the other nitrogen wastes. Ammonia is still the most energetically economical nitrogen waste to be synthesized.

In males the sexual activity is regulated by the endocrine glands hypophysis (pituitary), adrenals and gonads (testicles). The FSH (follicle-stimulating hormone) secreted by the adenohypophysis acts upon the testicles stimulating the spermatogenesis. The LH (luteinizing hormone), another adenohypophyseal hormone, stimulates the production of testosterone by the testicles too. Testosterone, whose production intensifies after the beginning of puberty, acts in several organs of the body and it is responsible for the appearing of the male secondary sex characteristics (beard, body hair, deep voice, increase of the muscle and osseous mass, maturation of genitalia, etc.) Testosterone also stimulates spermatogenesis.

The mentioned individual will produce eight different types of gametes (attention, gametes and not zygotes). To determine the number of different gametes produced by a given multiple genotype the number of heterezygous pairs is counted (in the mentioned case, three) and the result is placed as an exponent of two (in the example, 2³ = 8).

The factors that limit the growth of a population can be divided into biotic factors and abiotic factors. The main abiotic limiting factors are: availability of water and light, availability of shelter. The main limiting biotic factors are: population density and inharmonious (negative) ecological interactions (competition, predatism, parasitism, ammensalism). 

Nematodes are also known as roundworms. As the name indicates they are not flat like platyhelminthes. In evolutionary grounds with the nematodes the first complete digestive system appears, with mouth and anus, and the pseudocoelom is also a novelty.

Urea is more water-soluble than uric acid (an almost insoluble substance). Urea is also more toxic. Both however are less toxic than ammonia. Some invertebrates, chondrichthian fishes, adult amphibians and mammals are ureotelic. Reptiles, birds and most arthropods are uricotelic.

The organs that constitute the female reproductive system are the ovaries, the Fallopian tubes (or uterine tubes), the uterus, the vagina and the vulva.

Parental genotypes: AABB, aaBB. Gametes from the parental generation: Ab and aB. Thus F1 will present 100% AaBb gametes (and the phenotypical correspondent form).

As F1 are AaBb individuals the gametes from their crossing can be: AB, Ab, aB and ab. The casual combination of these gametes forms the following genotypical forms: one AABB, two AABb, two AaBb, four AaBB, one Aabb, one Aabb, one aaBB, two aaBb and two aabb. The phenotypical proportion then would be: nine A_B_ (double dominant); three A_bb (dominant for the first pair, recessive for the second); three aaB_ (recessive for the first pair, dominant for the second); one aabb (double recessive).

A typical population growth curve (number of individuals x time, linear scale) has a sigmoidal shape. There is a short and slow initial growth followed by a fast and longer growth and again a decrease in growth preceding the stabilization or equilibrium stage.

The population growth according to the biotic potential curve however is not sigmoidal, it is only crescent-shaped and points up to the infinite value of the scale (there is neither a decreasing stage nor equilibrium). 

Nematodes, like annelids, have a cylindrical elongated body. Annelids differentiate from nematodes by presenting a segmented body (body divided into metameres) and so they are called segmented worms.

Since amphibian larvae are aquatic they excrete ammonia. The terrestrial adult excretes urea.

The organ that liberates the female gamete is the ovary, the female gonad. The releasing of the oocyte is a response to hormonal stimuli. The immature egg cell (still an oocyte) falls into the abdominal cavity and is picked up by the Fallopian tube (uterine tube, or oviduct), a tubular structure that connects the ovary with the uterus.

Taking as example the crossing of AaBbCc with aaBBCc, for each considered pair of allele it is possible to verify which genotypes it can form (as in an independent analysis) and in which proportion. AA x aa: Aa, aa (1:1). Bb x BB: BB, Bb (1:1). Cc x Cc: CC, Cc,

cc (1:2:1). The genotype to which the probability is to be determined is for example aaBbcc. For each pair of this genotype the formation probability is determined: to aa, 0.5; to Bb, 0.5; to cc, 0.25. The final result is obtained by multiplication of these partial probabilities, 0.5 x 0.5 x, 0.5, resulting 0.0625.

Environmental resistance is the action of limiting abiotic and biotic factors that disallow the growth of a population as it would grow according to its biotic potential. Actually each ecosystem is able to sustain a limited number of individuals of a given species. The environmental resistance is an important concept of population ecology. 

Just like platyhelminthes, nematodes are triploblastics, i.e., they present three germ layers (ectoderm, mesoderm and endoderm).

Urea is a product of the degradation of amino acids. In the process amino acids lose their amine group which is then transformed into ammonia. In the liver ammonia reacts with carbon dioxide to form urea and water, a process called ureogenesis. In the intermediary reactions of the ureogenesis a molecule of ornithine is consumed and another is produced. For this reason ureogenesis is also known as the ornithine cycle.

The meiosis that forms female gametes begins in the cells of the ovarian follicles before birth. After the beginning of puberty, under hormonal stimuli, during each menstrual cycle one of the cells is released on the surface of the ovary and meiosis resumes. The meiotic process is only concluded however if fecundation happens.

This condition is called lack of dominance and it can happen in two ways: incomplete dominance or codominance.

In incomplete dominance the heterozygous presents an intermediate phenotype between the two types of homozygous, as in sickle cell anemia in which the heterozygous produces some sick red blood cells and some normal red blood cells. Codominance occurs, for example, in the genetic determination of the MN blood group system, in which the heterozygous has a phenotype totally different from the homozygous, not being an intermediate form.

The availability of water and light and the climate are abiotic factors that limit the growth of a population. Since the producers are responsible for the synthesis of organic material transferred along the food chains of an ecosystem, water and light affect the availability of food and a population cannot grow beyond the number of individuals the environment is able to feed. For example, in the desert, the biomass is relatively small and populations that live in this ecosystem are smaller (compared to the same species in environments with large available biomass). The climate, including the temperature, affects the population growth because excessive change in this factor, as the occurrence of droughts or floods, may cause significant population decline; small climatic changes can also alter the photosynthesis rate and reduce the availability of food in the ecosystem. 

The main evolutionary innovation of nematodes is the complete digestive system, with two openings (mouth and anus).

Since the ingestion and the defecation processes can occur in different extremities of the digestive tube, beings with a complete digestive system have the advantage of ingesting new food while residuals of already eaten food are still inside the body and not yet eliminated.

In reptiles and birds the excretory system is uricotelic since uric acid is insoluble, less toxic and suitable to be stored within the eggs where their embryos develop.

The external female genitalia is called the vulva. The vulva is the external opening of the vaginal canal, or vagina. The vagina is the copulation organ of the females and its posterior extremity communicates with the uterus through the uterine cervix. The uterus is divided into two portions: the cervix and the uterine cavity. The lateral walls of the uterine fundus communicate with the Fallopian tubes. The other extremity of each Fallopian tube ends in fimbria forming fringes in the abdominal cavity. Between the uterine tube and the ovary there is still intra-abdominal space.

There are many types of inheritance that do not follow the mendelian pattern. Notable among them are: multiple alleles, gene interactions (complementary genes, epistasis and quantitative, or polygenic, inheritance), linkage with or without crossing over and sex-linked inheritance.

Pleiotropy, lacking of dominance and lethal genes do not fit as variations of inheritance since genes can have these behaviors and at the same time obey mendelian laws.

Mutations and aneuploidies are abnormalities that also alter the mendelian pattern of inheritance as well as mitochondrial inheritance (passage of mitochondrial DNA from the mother through the cytoplasm of the egg cell to the offspring).

The difference between the real population growth curve (number of individuals x time) and the population growth according to the biotic potential curve of a given population is a result of environmental resistance. 

The cylindrical shape of nematodes made impossible the respiration exclusively by simple diffusion among cells since there are tissues far from the exterior. This problem was solved by the presence of an inner cavity in the body filled with fluid, the pseudocoelom. The pseudocoelom has the function of distributing gases and nutrients to the body and to collect residuals, besides serving as a hydrostatic base to keep the worm shape.

(For the fact that the pseudocoelom fluid and the pseudocoelom do not characterize a true circulatory system with blood and heart it is not said that in nematodes the respiration is cutaneous; it is considered that these animals still make respiration by diffusion).

The excretory system is formed of kidneys (two), ureters (two), bladder and urethra.

The menstrual cycle is the periodic succession of interactions between hormones and the organs of the female reproductive system that, after the beginning of puberty, regulates the release of the female gametes and prepares the uterus for fecundation and pregnancy.

Pleiotropy (or pliotropy) is the phenomenon in which a single gene conditions several different phenotypical traits.

Some phenotypical traits may be sensitive to pleiotropic effects (for example, inhibition) of other genes, even when conditioned by a pair of alleles in simple dominance. In these cases a mixture of pleiotropy and gene interaction is characterized.

Whenever a predator population increases at the first moment the prey population tends to decrease. At a second moment the decrease of the prey population and the bigger population density of predators cause the predator population to decrease. The prey population then reverts the tendency to decrease and begins to grow.

If variations in the size of populations occur in an unexpected intensity (different from the usual intensity of the ecological interaction) for example, due to ecological accidents killing many prey, the prey-predator equilibrium is disturbed and both species can be harmed. The existence of the predator sometimes is fundamental for the survival of the prey population, since the absence of predatism favors the proliferation of the prey and, in some cases, when the excessive proliferation creates a population size over the sustenance capacity of the ecosystem, environmental degradation occurs and the entire prey population is destroyed. 

The metabolic residuals of nematodes are collected by two longitudinal lateral excretory channels that open in one single excretory pore near the mouth. 

Placental animals, including embryos, excrete urea. In the adult placental mammal urea is excreted through the urine. In embryos the molecule passes to the mother’s blood through the placenta and it is excreted in the mother’s urine.

The maturation of the ovarian follicles after menses is stimulated by the action of FSH (follicle stimulating hormone).

Lethal genes are genes having at least one allele that, when present in the genotype of an individual, causes death. There are recessive lethal alleles and dominant lethal alleles. (There are also genes having alleles that are dominant when in heterozygosity but lethal when in homozygosity, i.e., the dominance related to the phenotype does not correspond to the dominance related to lethality.)

The growth curve according to the biotic potential of virus and bacteria both present a positive exponential pattern. The difference between them is that in each time period bacteria double their population while the viral population multiplies dozens or hundreds of times. The viral population growth curve thus has more intense growth. This happens because bacteria reproduce by binary division, each cell generating two daughter cells, while each virus replicates generating dozens or even hundreds of new viruses. 

Roundworms have a ganglial nervous system with an anterior neural ring representing (evolutionarily) a primitive cephalization.

Nematodes have two main longitudinal ganglial chords that extend one dorsally and the other ventrally under the epidermis. There may also be nerves lateral to these main chords. The nervous system of a free living nematode, “Caenorhabditis elegans”, has been well-studied in neurophysiological research and presents 302 neurons.

The nematode “C. Elegans” was the organism used in the research on the genetic regulation of organogenesis and apoptosis whose researchers won the Nobel prize of Medicine in 2002 (Brenner, Horvitz and Sulston).

The arterial vessels that carry blood to be filtrated by the kidneys are the renal arteries. The renal arteries are ramifications of the aorta and so the blood filtered by the kidneys is arterial (oxygen-rich) blood.

The endocrine glands that secrete hormones involved in the menstrual cycle are the hypophysis (pituitary) and the ovaries. The hormones from adenohypophysis are FSH (follicle-stimulating hormone) and LH (luteinizing hormone) and the hormones from the ovaries are estrogen and progesterone.

Gene interaction is the phenomenon in which a given phenotypical trait is conditioned by two or more genes (do not confuse with multiple alleles in which there is a single gene having three or more alleles).

The three main types of gene interaction are: complementary genes, epistasis and polygenic inheritance (or quantitative inheritance).

Age pyramids are graphical representations in form of superposed rectangles each representing the number of individuals included in age ranges into which a population is divided. Generally the lower age ranges are represented more to the bottom of the pyramid, always below higher ranges, and the variable dimension that represents the number of individuals is the width (there are age pyramids however in which the variable dimension is the height).

Nematodes reproduce sexually. The nematode sperm cell does not have cilia nor flagella and they move by amoeboid movement forming pseudopods.

Human beings excrete mainly urea eliminated with the urine.

The follicles that are growing after menses secrete estrogen. These hormones act upon the uterus stimulating the thickening of the endometrium (the internal mucosa of the uterus).

Multiple alleles is the phenomenon in which the same gene has more than two different alleles (in normal mendelian inheritance the gene has only two alleles). Obviously these alleles combine in pairs to form the genotypes.

In multiple alleles relative dominance among the alleles may exist. A typical example of multiple alleles is the inheritance of the ABO blood group system, in which there are three alleles (A, B or O, or IA, IB and i). IA is dominant over i, which is recessive in relation to the other IB allele. IA and IB lack dominance between themselves.

Another example is the color of rabbit fur, conditioned by four different alleles (C, Cch, Ch and c). In this case the dominance relations are C > Cch > Ch> c (the symbol > means “dominates over”).

In a stabilized human population the age pyramid has a narrower base since the reproduction rate is not so high. The adult age ranges are generally wider than the infantile ranges showing that in practice there is no population growth. There is a proportionally high number of older individuals meaning that the life quality is elevated and the population has access to health services and good nutrition. These are features of the age pyramids of developed countries. 

The main human diseases caused by nematodes are ascariasis, ancylostomiasis (hookworm infection) and filariasis (commonly known by its consequence, elephantiasis).

The venous vessels that collect the blood filtered by the kidneys are the renal veins. The renal veins carry the blood that has been reabsorbed in the nephron tubules.

By convention the menstrual cycle begins at the day that menses begins. (Menses is the endometrial hemorrhage excreted through the vaginal canal.) At these days the hormones FSH, LH,  estrogens and progesterone are in low concentration.

The study of human age pyramids can provide the following analyses: proportion of individuals at an economically active age; proportion of elderly (indicating the quality of the pension and health systems); proportion of children and youth (indicating need for job generation and educational services); reproductive profile (shows the population growth tendency); postnatal survival rate (indicates quality of the health system, hygiene conditions, nutrition and poverty); longevity profile; etc.

It is possible to predict whether a population belongs to a rich and industrialized society or to a poor country since the patterns of the age pyramids differ according to these conditions. 

Examples of representing beings: ascaris, hookworms, filaria, pinworms. Basic morphology: cylindric (round) body, not segmented.

Type of symmetry: bilateral.  Germ layers and coelom: triploblastics, pseudocoelomates.

Digestive system: complete. Respiratory system: respiration by diffusion.

Circulatory system: circulating fluid within the pseudocoelom. 

Excretory system: excretory channels and excretory pore. 

Nervous system: ventral and dorsal ganglial chord, primitive cephalization. Types of reproduction: sexual.

The functional (filtering) unity of the kidneys is the nephron. A nephron is made of afferent arteriole, efferent arteriole, glomerulus, Bowman's capsule, proximal tubule, loop of Henle, distal tubule and collecting duct. In each kidney there are about one million nephrons.

The increase in the blood concentration of estrogen with the growing of the ovarian follicle causes the hypophysis to secrete LH. In this phase LH acts together with FSH promoting the maturation of the follicle that at the 14th day ruptures releasing the female gamete (ovulation). After the release of the ovum LH acts stimulating the formation of the corpus luteum, a structure made from the remaining follicular mass. The LH concentration is at maximum at the 14th day of the cycle.

Epistasis is the gene interaction in which a gene (the epistatic gene) can disallow the phenotypical manifestation of another gene (the hypostatic gene). In dominant epistasis the inhibitor allele is the dominant allele (for example, I) of the epistatic gene so inhibition occurs in dominant homozygosity (II) or in heterozygosity (Ii). In recessive epistasis the inhibitor allele is the recessive allele of the epistatic gene (i) so inhibition occurs only in recessive homozygosity (ii).

The age pyramids of peripheral countries or underdeveloped countries have characteristics related to the poverty of such populations, with a wider base and narrow apex. The base age range, if much wider than the other levels, indicates a high birth rate. The levels just above the base may present an impressive reduction in poorer populations due to infant mortality.

Ranges that represent the youth are also wide showing future pressure on job and habitation needs. The widths of the rectangles diminish as age increases to the apex that represents the elderly, demonstrating difficult life conditions, precarious health services and low life expectancy.

Earthworms, leeches and lugworms.

If fecundation does not occur approximately 24 hours after ovulation the released ovum often dies.

Pollution is the contamination of an ecosystem by factors that are harmful for the equilibrium of its biotic or abiotic constituents. 

Platyhelminthes are worms with flat bodies (flatworms), nematodes are worms with cylindrical but not segmented bodies (roundworms). Annelids are cylindrical worms with segmented bodies (they are metameric).

Glomerular filtrate is the name given to the plasma after it has passed the glomerulus and entered the Bowman’s capsule. The glomerular filtrate has a different composition compared to urine since the fluid has not yet undergone tubular resorption and secretion. The main difference between the blood and the glomerular filtrate is that in the latter the amount of proteins is at a minimum and there are no cells or blood platelets.

The hormones that promote the release of the ovum from the follicle are FSH and LH, hormones found in maximum blood concentration around the 14th day of the cycle. The release of the female gamete from the ovary is called ovulation. Ovulation happens at (around) the 14th day of the menstrual cycle.

In most cases pollution is caused by human activity. Other species and some abiotic factors however can also pollute an ecosystem. For example, the red tide is created by proliferation of some algae and volcanic dust is a consequence of the internal activity of the planet.

The main evolutionary novelty presented by the beings of the phylum Annelida is the coelom, the internal body cavity totally covered by mesoderm, a feature also present in arthropods, molluscs, echinoderms and chordates. Platyhelminthes are acoelomate and nematodes are pseudocoelomate (their internal cavity is partially covered by mesoderm).

Another important evolutionary novelty of the annelids is the closed circulatory system.

Proteinuria means losing of proteins through urine. Under normal conditions proteins are too big to be filtered by the glomerulus and they are practically absent in the urine (the few filtered proteins may also be resorbed in the nephron tubules). Proteinuria is an indication that a more than expected amount of proteins is passing the glomerulus suggesting glomerular disease, e.g., in diabetic nephropathy. The glomerulus also blocks the passage of blood cells and platelets (hematuria is often a sign of urinary disease although less specific of kidneys since the blood may come from the lower parts of the excretory tract).

The follicle that released the ovum suffers the action of LH and is transformed into the corpus luteum. The corpus luteum is very important because it secretes estrogen and progesterone.

These hormones prepare the uterine mucosa, also known as endometrium, for nidation (implantation of the zygote in the uterine wall) and embryonic development since they stimulate the thickening of the mucous tissue, increase its vascularity and make the appearing of uterine glycogen-producing glands.

Considering “p” the number of phenotypical forms and “a” the number of involved alleles of the polygenic inheritance. The formula p = 2a + 1 is then applied.

(Many times it is not possible to determine precisely the number of phenotypical forms, p, due to the multigenic feature of the inheritance, since often the observed variation of phenotypes seems to be on a continuum or the trait suffers environmental influence.)

The environmental problem concerning waste worsens with industrial development and the global growth of consumption societies in the 20th and 21st centuries, factors that cause the immense volume of residuals produced by mankind in the last decades. The increased waste generation raises the issue about what to do with waste since nature is not able to degrade and resorb with adequate speed and efficiency most of the residuals. Therefore the various kinds of waste accumulate, polluting the environment and creating danger to humans and nature. (The present destination of waste has been public waste depositories where the waste volume is compressed and buried underground, an environmentally risky method. Another method has been incineration, with the grave consequence of causing air pollution.) 

The metameric feature, i.e., the body segmentation in metameres, approximates annelids to arthropods since these animals are segmented beings too. (Bristles present in oligochaete and polychaete annelids are also covered with chitin, the same substance of the arthropod exoskeleton.)

Only 0.5 to 1% of the glomerular filtrate is eliminated as urine. The remaining volume, containing mainly metabolic ions, glucose, amino acids and water, is resorbed through the nephron tubules (by means of active or passive transport) and gains the blood circulation again. The convolute tubules of the nephron are responsible for the resorption of substances.

The female gamete released from the ovary falls into the surrounding abdominal cavity and is collected by the Fallopian tube. The internal epithelium of the uterine tubes has ciliated cells that move the ovum or the fecundated egg cell towards the uterus.

If a trait statistically has a normal (gaussian, bell-shaped curve) distribution of its phenotypical forms it is probable that it is conditioned by polygenic inheritance (quantitative inheritance).

In quantitative inheritance the effects of several genes add to others making it possible to represent the trait variation of a given population in a gaussian curve with the heterozygous genotypes in the center, i.e., appearing in larger number, and the homozygous in the extremities.

Digestion in beings of the phylum Annelida is extracellular. These animals have a complete digestive system, with mouth and anus.

The cells of the tubule wall have high number of mitochondria because many substances are resorbed or secreted through them by means of active transport (a process that spends energy). Therefore many mitochondria are necessary for the energetic supply (ATP supply) of this type of transport.

The uterine glands produce glycogen that can be degraded into glucose to nourish the embryo before the complete development of the placenta.

Sex-linked inheritance is a type of nonmendelian inheritance because it opposes Mendel’s first law, which postulates that each trait is always conditioned by two factors (alleles). In nonhomologous regions of the sex chromosomes the genotypes of the genes contain only one allele (even in the case of the XX karyotype, i.e., in women, one of the X chromosomes is inactive).

Recyclable waste is waste that can be reprocessed and used again. Waste recycling depends on the separation of the recyclable residuals from non recyclable ones and on the classification of the recyclable into plastics, metals, papers, etc. The function of the selective waste collection is to simplify that separation for the waste to be sorted at the point of origin. Selective collection also helps the creation of an environmental conscience in the people that produce the waste.

Earthworms eat decomposing organic material and small organisms ingested together with soil particles. The digestive tubes of earthworms have special structures, like a muscular wall and a gizzard, that triturate the food and scratch it against the ingested soil particles. Since annelid digestion is exclusively extracellular earthworms also present in the posterior part of their digestive system structures like the cecum and the typhlosole that have the function of increasing the absorption surface of the intestine.

Tubular secretion is the passage of substances from the blood capillaries that surround the nephron tubules to the tubular lumen for these substances to be excreted with urine. Ammonia, uric acid, potassium, bicarbonate and hydrogen ions, metabolic acids and bases, various ingested drugs (medicines) and other substances are secreted by the nephron tubules.

After ovulation the estrogen and progesterone secretions from the corpus luteum inhibit the hypophyseal FSH and LH secretions (this happens by inhibition of GnRH, gonadotropin-releasing hormone, a hypothalamic hormone). The blood concentration of these adenohypophyseal hormones falls to basal levels again. As LH lowers the corpus luteum (luteum means “yellow”) becomes atrophic and turns into the corpus albicans (“white”). With the regression of the corpus luteum the production of estrogen and progesterone ceases.

Mitochondrial inheritance is the passage of mitochondrial DNA molecules (mtDNA) to the offspring. All stock of mtDNA an individual has have come from the mother, the maternal grandmother, the maternal great grandmother and so on, since mitochondria are inherited from the cytoplasm of the egg cell (that later constitutes the cytoplasm of the zygote). There are several genetic diseases caused by mitochondrial inheritance, like Leber's hereditary optic neuropathy, that leads to loss of the central vision of both eyes, and the Kearns-Sayre syndrome, a neuromuscular disease that causes ophthalmoplegia and muscle fatigue. Mitochondrial inheritance is an excellent means of genetic analysis of the maternal lineage (just like the Y chromosome is an excellent means of study of the paternal lineage).

To treat sewage is much cheaper for society. The non treated sewage pollutes rivers, lakes and the sea, being a cause of diseases transmitted through water. For the society the costs of these diseases are much higher than the cost of the sewage treatment.

One of the most economical systems to treat sewage is the aerobic treatment system, reservoirs kept very oxygenated for aerobic bacteria to decompose organic material. 

Ingested blood does not coagulate inside the leech (Hirudo medicinalis) because in its saliva there is a potent anticoagulant substance, a protein called hirudin.

In the past leeches were largely used as medical treatment. Nowadays hirudotherapy is being used in patients with extensive and chronic inflammation of the skin, in prevention against tissue necrosis after some surgeries and in several others fields of Medicine.

The regulation of the acid-basic equilibrium of the body is done by the kidneys and depends upon the tubular resorption and secretion.

The menstrual cycle is divided into two main phases: the follicular (or menstrual) phase and the luteal (or secretory) phase. The menstrual phase begins at the first day of menses and lasts until ovulation (around the 14th day). The luteal phase begins after ovulation and ends when menses begins (around the 28th day).

Two genes are said to be under linkage, or linked, when they reside in the same chromosome.

For example, the research of the human genome discovered that the factor III of clotting gene and the factor V of clotting gene are located in the same chromosome (the human chromosome 1). The factor VII gene however is not linked to those genes since it is located in the chromosome 13.

Eutrophication is the process of excessive increasing of nutrients, like phosphate and nitrate, in water due to direct deposit of non treated sewage. The nutrients act as fertilizers leading to abnormal proliferation of aquatic algae. With the exaggerated growth of the alga population the number of aerobic bacteria that cause decomposition of organic material also increases. The proliferation of these bacteria depletes the dissolved oxygen killing fishes and other animals. Besides, the lack of oxygen causes the decomposition to be assumed by anaerobic bacteria.

Anaerobes multiply and release hydrogen sulfide that makes water improper to other living beings and creates a putrid smell.

Respiration in annelids can be cutaneous or branchial. Cutaneous respiration occurs due to the rich vascularity under the epidermis. The gills, present in aquatic annelids, are located in the parapodia (false claws) that have an extensive capillary net.

Kidneys can regulate the acidity or alkalinity of the plasma varying the excretion of hydrogen and bicarbonate ions. In alkalosis (abnormally high level of the plasma pH) the kidneys excrete more bicarbonate and the equilibrium of formation of bicarbonate from water and carbon dioxide shifts towards formation of more hydrogen ions and bicarbonate and then the plasma pH is lowered. When the body undergoes acidosis (abnormal low level of the plasma pH) the kidneys excrete more hydrogen ions and retain more bicarbonate thus the equilibrium of formation of bicarbonate from water and carbon dioxide shifts towards more hydrogen consumption and the plasma pH is increased.

Menses is the endometrial monthly desquamation that occurs as the estrogen and progesterone levels fall after the regression of the corpus luteum because these hormones, mainly progesterone, can no longer support and maintain the thickening of the endometrium.

Mendel’s second law, or the law of the independent assortment, is valid for genes located in different chromosomes. These genes during meiosis segregate independently.

Mendel’s second law however is not valid for phenotypical features conditioned by genes located in the same chromosome (genes under linkage), since these genes, known as linked genes, do not separate in meiosis (except for the phenomenon of crossing over).

Examples of other heavy metals that cause toxic pollution are lead, cadmium and chromium. 

The classification of these beings as vascular beings means that they have a circulatory system, with vessels that distribute substances throughout the body.

Poriferans, cnidarians and flatworms do not have a circulatory system. In nematodes there is circulation of gases and nutrients through the pseudocoelom fluid.

The kidneys and the hormones that act upon them are the main physiological regulators of the total blood volume of the body. As more water is resorbed in the nephron tubules the more the blood volume increases; as more water is excreted in urine the more the blood volume lowers. The blood volume in its turn has a direct relation to blood pressure. The blood pressure increases when the blood volume increases and it lowers when the blood volume lowers. That is the reason why one of the main groups of antihypertensive drugs is the diuretics. Doctors often prescribe diuretics for the hypertensive patients to excrete more water and thus lower their blood pressure.

The hemorrhage that accompanies menses occurs because the endometrium is a richly vascularized tissue. The rupture of blood vessels of the uterine mucosa during the menstrual desquamation causes the bleeding.

The fruit fly drosophila is suitable for the study of Genetics because it presents many distinct traits but only four chromosomes (one sex chromosome and three autosomes).

A biodigester is equipment that produces carbon dioxide, hydrogen sulfide and fuel gases (biogases) like methane from organic material under decomposition (dung, food waste, sugar cane waste, etc.). The biogas is used in heating, as energy for motors and machines and it even has industrial uses. Biodigesters are widely used in public waste depositories and in rural areas. Besides producing biogas the organic waste can be turned into good quality fertilizer. 

A circulatory system is classified as open or closed. In open circulatory systems blood gets out of vessels and flows also to large cavities that perfuse the tissues to be irrigated. In closed circulatory systems blood circulates only within blood vessels and through the heart.

Antidiuretic hormone (or ADH, or vasopressin), aldosterone and atrial natriuretic factor (or ANF) are hormones that participate in the regulation of the excretory system.

One can imagine a cycle like an analog clock at which at 0 o’clock is the beginning and the end of the menstrual cycle and that 6 o’clock corresponds to the 14h day of the cycle. At 0 o’clock the menses and so the menstrual cycle begins and FSH blood level begins to increase. Around 2 o’clock the maturing follicles under FSH action are already secreting estrogen and the endometrium is thickening. Around 3 o’clock estrogen is intensely stimulating the increase of LH blood level. At 6 o’clock (the 14th day) LH is at its maximum concentration and FSH also at high levels to promote ovulation, LH then stimulates the formation of the corpus luteum. Around 7 o’clock the corpus luteum is already secreting a great amount of estrogen and progesterone and the endometrium thickens even more, concomitant lowering of FSH and LH occurs with the increasing of the ovarian hormones. Around 11 o’clock the reduced LH and FSH levels make the corpus luteum turn into the corpus albicans, the production of estrogen and progesterone ceases and the endometrium regresses. At 0 o’clock again (28th day) the endometrium desquamates and a new menstrual cycle begins.

POPs, or persistent organic pollutants, are toxic substances formed from organic compounds. POPs are made in several industrial processes, like the production of PVC, paper whitened by chlorine, herbicides, insecticides and fungicides, and also in the incineration of waste. Examples of POPs are dioxins, furanes, chlordane, DDT, dieldrin, heptachloride, toxaphen and hexachlorbenzene.

POPs are toxic and highly harmful since, like the heavy metals, they are bioaccumulative, i.e., they are not degraded by the body and accumulate even more in each following trophic level of the food chains. In humans POPs can cause cancer and nervous, immune and reproductive impairments. 

In beings of the phylum Annelida the circulatory system is closed, i.e., blood circulation takes place only within specialized vessels.

The antidiuretic hormone is secreted by the hypophysis (also known as pituitary) and it acts in the nephron tubules increasing the resorption of water. When the body needs to retain water, for example, in cases of blood loss and abrupt blood pressure lowering or in cases of abnormally high blood osmolarity, there is stimulus for ADH secretion. When the body has an excess of water, as in cases of excessive ingestion or in abnormally low blood osmolarity, the secretion of ADH is blocked and the diuresis increases. ADH is also known as vasopressin since it increases the blood volume and thus heightens the blood pressure.

Fecundation generally occurs in the Fallopian tubes but it can also take place within the uterus. There are cases when fecundation may occur even before the ovum enters the uterine tube, a fact that may lead to a severe medical condition known as abdominal pregnancy.

The farther the distance between the loci of two genes in a chromosome the higher the recombination frequency between these genes. This is true because once alleles are nearer in the chromosome it is more probable that they are kept united when chromosomal extremities are exchanged by crossing over. On the other hand, if they are farther apart it will be easier for them to separate by crossing over.

Mercury is a metal that when present in the water of rivers, lakes and seas contaminates fishes, crustaceans, molluscs and other living beings. The mercury accumulates along the food chain and in each following trophic level the amount of the metal within the individuals is higher. When humans eat contaminated animals they also become contaminated and severe nervous system injuries may emerge. The main sources of mercury pollution are gold mining and the use of derived substances in industry and agriculture. 

The blood in beings of the phylum Annelida contains the respiratory pigment hemoglobin (the same found in chordates) and other pigments too.

Alcohol inhibits the secretion of ADH (antidiuretic hormone) by the pituitary. That is why when it is drunk to excess the person urinates too much.

During sexual arousal in women the vagina secretes substances to neutralize its acidity thus allowing the survival of sperm cells within it. During the female fertile period hormones make the mucus that covers the internal surface of the uterus less viscous to help the passage of sperm cells to the uterine tubes. During copulation the uterine cervix advances inside the vagina to facilitate the entering of male gametes through the cervical canal.

Parental gametes are those gametes that maintain the original linkage of genes (alleles) in the chromosome. Recombinant gametes are those in which the original linkage is undone due to exchange of chromosomal pieces by crossing over during meiosis.

The upward movement of warm air is a natural method of dispersion of pollutants. The air near the ground is hotter because the sun heats the soil and the soil heats the air nearby. Since it is less dense, the warm air tends to move towards higher and colder strata of the atmosphere. Such movement helps the dispersion of pollutants.

In these beings there are a longitudinal muscular layer under the epidermis and, internally juxtaposed and perpendicular to it, another circular (radial to the axis) muscular layer. The circular muscle layer has the function of elongating the body while the longitudinal shortens it. By alternating actions both promote movement.

The renal regulator hormone secreted by the heart is the atrial natriuretic factor (or ANF). The ANF increases the excretion of sodium in the nephron tubules causing less resorption of water, more urinary volume, and thus lowering the blood pressure. The atrial natriuretic factor is secreted when there is an increase of the length of the heart muscle fibers in response to high blood pressure. The ANF is a natural antihypertensive substance. Since the health of the heart depends largely upon the stability of the normal blood pressure the evolution should have preserved the atrial natriuretic factor to allow information from the heart to be an additional mechanism for the renal control of the blood pressure.

Although this is not a rule, to be effective fecundation in general must occur within about 24 hours after ovulation (that occurs around the 14th day of the menstrual cycle). Fecundation may occur even if copulation took place up to 3 days before ovulation since the male gametes remain viable for about 72 hours within the female reproductive system. The fertile period of the women however is considered the period from 7 days before ovulation to 7 days after ovulation.

Centimorgan, or recombination unit, by convention is a distance between two linked genes that corresponds to 1% of recombination frequency of these genes.

Ozone, O3, is a gas of the atmosphere that filters ultraviolet radiation from the sun disallowing most of that radiation from reaching the surface of the planet. Ultraviolet radiation is harmful for living beings because it is a mutagen and can cause cancer (mainly skin cancer), other DNA mutations and even burns. 

In each segment (metamere) of the being a pair of complete excretory structures called metanephridium exists. The metanephridium has an extremity, the nephrostoma, which collects residuals from the coelom, filtering them and causing reabsorption along its extension (similar to human nephron tubules). The material to be excreted goes out through a pore, the nephridiopore, which opens in the body surface.

Alcohol inhibits the secretion of ADH (antidiuretic hormone) by the pituitary. That is why when it is drunk to excess the person urinates too much.

Nidation is the implantantion of the embryo in the uterus. Nidation occurs around the 7th day after fecundation, i.e., 7 to 8 days after ovulation (obviously, it occurs only if fecundation also occurs). Since it occurs in the luteal phase the progesterone level is high and the endometrium is in its best condition to receive the embryo.

Recombination frequency, or crossing over rate, is the percentage of recombinant gametes made by crossing over (in relation to the number of parental gametes made). It always refers to two genes located in the same chromosome.

Pollutant low altitude thermal inversion occurs in the winter. In this period of the year the sun heats the soil less and the natural upward move of warm air decreases. Therefore the pollutants form a low altitude layer between the cold air layer near the ground and another layer of warmer air above. The pollutant layer over industrial areas or big urban concentrations reduces the penetration of the sun's energy and the air bellow takes an even longer time to warm. 

Annelids have a nervous system made of two ventral chords and one relatively big nervous cell concentration in its anterior portion resembling a primitive brain.

Nematodes have an anterior neural ring connected to two neural chords, a ventral and a dorsal one, while in planarias (platyhelminthes) there are only two small anterior “cerebral” ganglia from which neural chords split. Cephalization in annelids thus is more outstanding than in nematodes or in flatworms.

Aldosterone is a hormone that acts upon the nephron tubules stimulating the resorption of sodium. Therefore it contributes to the increase of the blood osmolarity and consequently to the increase of the blood pressure. Aldosterone is made by the adrenals, glands located over the superior portion of the kidneys.

Many times fecundation takes place in the Fallopian tubes. Generally the newly formed zygote is taken to the uterus where nidation and the embryonic development occur. In some cases however the zygote cannot go down to the uterus and the embryo implants itself in the uterine tube tissue, characterizing the tubal pregnancy. Tubal pregnancy is a severe clinical condition since often the tube ruptures during gestation causing hemorrhage and even death of the woman. The most common treatment for tubal pregnancy has been surgery.

Genetic mapping is the determination of the location of the genes in a chromosome. By determining the recombination frequency between several different linked genes it is possible to estimate the distance between them in the chromosome. For example, if a gene A has a recombination frequency of 20% with the gene B, this gene B has recombination frequency of 5% with the gene C and this gene C has recombination frequency of 15% with the gene A, it is possible to assert that the gene A is 20 centimorgans distant from the gene B and that between them lies the gene C at 15 centimorgans of distance from the gene A.

The mains chemical compounds that destroy the ozone layer are the CFCs, chlorofluorocarbons, or freons, substances used in the past in refrigerators, airconditioners and spray cans. Chlorofluorocarbons react with ozone in the high atmosphere releasing molecular oxygen and therefore the amount of ozone in the atmosphere is reduced. Another substance that destroys the ozone layer is methyl bromide, used in agricultural insecticides.

The clitellum is a special region of the annelid constituted by rings (metameres) with reproductive function. It can be found in the anterior portion of the animal and it is characterized by a lighter color in comparison to the normal color of the other segments.

Hemodialysis is the artificial blood filtration made by specific machines in substitution of the kidneys. Hemodialysis may be necessary in patients suffering from diseases that cause renal failure, like diabetic renal complications, lupic renal complications and others. During hemodialysis the blood of the patient is deviated to the filtering machine and after the filtration it returns to the body. Hemodialysis is generally done two, three or more times a week in a process that takes several hours. Sometimes kidney transplantation is an alternative to hemodialysis.

Laboratory tests to detect pregnancy commonly test for human chorionic gonadotropin (HCG) concentration in blood or urine samples. If the level of this hormone is abnormally high, pregnancy is likely.

Sexual reproduction and recombination of linked genes (crossing over) are, along with mutations, the main instruments of biological variability. Sexual reproduction allows many combinations between genes situated in different chromosomes. Crossing over, however, is the only means to provide recombination of alleles located in a same chromosome. Crossing over probably emerged and has been maintained by the evolution because of its importance to biological diversity.

Thermal inversion confines at low altitude a layer of pollutants that would have been dispersed by the natural upward move of warm air. The solid particles present in the atmosphere cause health problems, like the exacerbation of asthma and other pulmonary diseases, cough, respiratory unease and ocular discharges; later the pollution can also trigger the appearance of cardiovascular and neoplastic diseases. 

Earthworms have an important ecological role as they eat decomposing organic material. They also dig tunnels in the subsoil allowing the entrance of gases and nutrients that are useful for plant roots and other living beings. So they act as decomposers and as fertilizers too.

The skin is the external covering of the body. In humans its main functions are protection, perception of information from the environment, control of the body temperature and secretion of substances.

The functioning of the hypophysis is altered during pregnancy. Since estrogen and progesterone levels remain elevated during the gestational period the production of GnRH (gonadotropin-releasing hormone) from the hypothalamus is inhibited. The lack of GnRH thus inhibits the secretion of FSH and LH from the hypophysis and a new menstrual cycle does not begin. If pregnancy does not occur the lowering of estrogen and progesterone levels stimulates the production of GnRH by the hypothalamus. This hormone then hastens the adenohypophyseal secretion of FHS and LH that in their turn stimulate the maturation of follicles and the beginning of a new menstrual cycle.

The individual of the male sex is XY so he forms gametes containing either the X chromosome or the Y chromosome in a 1:1 proportion. The individual of the female sex is XX and thus she forms only gametes containing an X chromosome.

Nuclear pollution consists of radiations emitted from atomic nuclei, these radiations are highly injurious to living beings. They can be originated from the extraction of radioactive minerals, nuclear plant reactors, nuclear research centers, hospitals and medical centers that use radioisotopes, nuclear bomb explosions or accidents with transportation, handling or storage of nuclear material. Nuclear materials remain dangerous for many years, contaminating the environment with radiation that can cause cancer, immune impairment, congenital deficiencies, burns and even death. The damage is proportional to the intensity of the exposition to the radiation. Its persistent feature and high aggression power make nuclear pollution one of the major environmental problems of our time. 

These beings may be dioecious (the majority of polychaetes) or hermaphrodite monoecious (oligochaetes and hirudineans).

The epithelial tissues can perform covering, impermeability and protection against the environment, for example, in the skin, resorption, as in the guts and renal tubules, gas exchange, for example, the amphibian skin, thermal regulation, like sweating, secretion of substances, as in the epithelium of glands. In some animals the skin also has the important function of camouflage and mimicry .

The placenta besides being the organ through which the exchange of substances between the mother and the fetus is done also has the function of secreting estrogen and progesterone to keep a high level of these hormones during pregnancy. (The placenta still secretes other hormones like human placental lactogen, that act similarly to the hypophyseal hormones that regulate reproduction, and HCG, human chorionic gonadotropin.)

In the diploid genome of human beings there are 46 chromosomes, 44 of them are autosomes and two are sex chromosomes. The individual inherits one of these chromosomes from each parent. The human sex chromosomes are called X chromosome and Y chromosome. Individuals having two X chromosomes (44 + XX) are female. Individuals having one X chromosome and one Y chromosome (44 + XY) are male. (Individuals 44 + YY do not exist since the chromosome Y is exclusively from paternal lineage.)

Transgenic beings are animals, microorganisms and plants that contain recombinant DNA, i.e., genes from other plants, microorganisms or animals artificially inserted into their genetic material. Transgenic beings are made for scientific and economic purposes, in this last case with the intention of improving their commercial features. For example, bacteria that produce human insulin are transgenic beings made by biotechnology. The main targets of the transgenic technology are edible vegetables, like soy, corn, potato and tomato. 

The phylum is divided into three classes: oligochaetes (for example, earthworms), hirudineans (e.g., leeches) and polychaetes (these are mostly marine aquatic with parapodia, like nereis).

The skin of vertebrates is made of epidermis, an external layer of epithelial tissue, and dermis, a layer of connective tissue under the epidermis. One can cite also the hypodermis, a layer of adipose tissue under the dermis. Skin annexes may exist in some phyla and classes, like hair, sweat glands and sebaceous glands.

Contraceptive pills generally contain the hormones estrogen and progesterone. If taken daily from the 4th day after menses the abnormal elevation of these hormones acts upon the hypophysis- hypothalamus endocrine axis inhibiting the FSH and LH secretions. Since these hormones then do not reach their normal high levels during the menstrual cycle ovulation does not occur ( Treatment with contraceptive pills must be initiated under medical supervision.)

Homologous portion is that in which there are genes having alleles in both Y and X sex chromosomes. The homologous portions are situated more in the central part of the sex chromosomes, near the centromere.

Plutonium is the highly radioactive chemical element produced from uranium by nuclear plants. Plutonium can be reprocessed to be used again in nuclear plants or in other destinations, like the making of nuclear bombs. Plutonium reprocessing nowadays, however, is done only in some countries like France, Russia and Britain. The countries that have nuclear plants, like Japan, Australia, etc., send their atomic waste by ship to those plutonium reprocessing centers. Besides the inherent risks of the storage of nuclear waste, plutonium reprocessing brings the risks of the transport of radioactive material across the oceans. The “nuclear ships” often travel near the coast of many countries posing danger to their populations. 

In earthworms there is no larval stage, so the embryonic development is direct.

The typical feature of the epithelium is the absence or almost absence of space between cells. The epithelial cells are compactly positioned side-by-side with the help of specialized structures for cell adhesion like desmosomes and interdigitations. This feature relates to the fact that these tissues are generally exposed to an exterior surround and so they need more resistance and impermeability against the entrance of strange material into the body. The connective tissue presents opposite features due to its filling function. It has much interstitial material (the matrix) and relatively large space between cells.

The contraceptive diaphragm is an artifact made of latex or plastic that when placed on the vaginal fundus covers the uterine cervix forbidding the passage of sperm cells through the cervical canal. To be more effective the diaphragm needs to be used together with spermicide. This method however does not prevent sexually transmitted diseases (STDs).

It is not only possible that an X chromosome of a woman is from her father, it is certain. Every woman has an X chromosome from her father and the other X chromosome from her mother.

In men however the X chromosome comes always from his mother and the Y chromosome is always from his father.

Transgenics can be dangerous to the entire biosphere since the transfer of genes between species may have immediate and long term unpredictable consequences. The creation of new species by nature is a slow process, dependent on causal mutations and natural selection, a relatively safe process for the ecological equilibrium. It is impossible to know how the fast and artificial introduction of transgenic beings in nature affects ecosystems.

Pathogenic agents may be involuntarily created in laboratories, spreading unknown diseases; transgenic species may uncontrollably proliferate destroying ecological interactions that have taken thousands of years to be established; the ingestion of transgenic food also has unpredictable effects.

Among the annelid classes only polychaetes present a larval stage. Their larva is called trocophore.

Besides the skin there are other covering tissues made of epithelium over other tissue layers. They are the tissues that cover the internal surfaces of hollow organs, like the organs of the digestive tube, the airway, the renal tubules, the ureters, the bladder, the urethra and the blood vessels. The glands and the serous membranes are made of epithelial tissue too.

There are medical reports associating the use of contraceptive pills with vomiting, nausea, vertigo, headaches, hypertension and other pathological conditions. Some research has attempted to relate the medical ingestion of estrogen and progesterone with increased propensity to cardiovascular diseases (like infarction, strokes and thrombosis) and to malignant neoplasias (cancers). Doctors must always be asked about the risks and benefits of the contraceptive pill prior to use.

To analyze the X DNA of a mother (assuming no access to her own material) it is more indicated to study the genetic material of her sons since all X chromosomes of males come from the mother while the daughters have X chromosomes from the mother and from the father. By researching the material of the sons it is ensured that the studied X chromosome is from the mother.

Biological control is a natural method to control the size of animal, microorganism or plant populations. Biological control is based on the knowledge of inharmonious (negative) ecological interactions between species. Using such knowledge a parasite, competitor or predator species is introduced in an ecosystem in order to attain reduction of the population of another species with which it has inharmonious ecological interaction. The biological control presents the advantage of substituting the use of pesticides and other toxic chemical products in the control of plagues and diseases. It however should be employed with caution under serious previous study to avoid harmful ecological disequilibrium. A kind of biological control of some species can be done by the introduction of previously sterilized males, that do not generate offspring. 

Examples of representing beings: earthworms, leeches, lugworms. Basic morphology: cylindrical body, segmented (metameric). Type of symmetry: bilateral. Germ layers and coelom: triploblastics, coelomates.

Digestive system: complete. Respiratory system: cutaneous or branchial.

Circulatory system: closed, with hemoglobin. Excretory system: a pair of metanephridia in each metamere.

Nervous system: neural chords, a pair of ganglia per metamere, anterior concentration of neurons (primitive brain). Types of reproduction: sexual, with dioecious and monoecious beings.

The structures responsible for the union of the epithelial cells are called cell junctions. The main cell junctions are interdigitations, desmosomes, zonula adherens (adherens junction), tight junctions (zonula occludens) and gap junctions.

The use of condoms besides being an efficient contraceptive method also helps the prevention of diseases caused by sexually transmitted agents (STDs), like syphilis, gonorrhea, HPV (human papilloma virus that may lead to genital cancers) infestation, HIV infection, etc.

Besides sex genes the sex chromosomes have also autosomal genes, genes that codify several proteins related to nonsexual traits.

Bioremediation is the use of microorganisms, like bacteria, protists and fungi, to degrade noxious substances turning them into non toxic or less toxic substances. Bioremediation employs microorganisms whose metabolism uses contaminants as reagents.

Bioremediation is used, for example, in the decontamination of environments polluted by oil spills. In this process bacteria that use hydrocarbons as substrate for their cellular respiration are employed.

Ants, flies, cockroaches, shrimps, crabs, spiders and scorpions are examples of arthropods.

Epithelia are not vascularized (capillaries do not directly reach their cells). The epithelium exchanges substances by diffusion with the connective tissue situated under it. Since the epithelia are not vascularized minuscule skin injuries or scratches that happen all the time do not trigger bleeding and do not expose the blood to contamination from external agents. This is an important protective strategy discovered by evolution.

Vasectomy is the most common method of surgical sterilization in men. In vasectomy the vas deferens inside the scrotum are sectioned and closed at a section which will forbid the sperm cells to follow to the ejaculatory duct but still allowing the release of seminal fluid during ejaculation.

Surgical sterilization of women is often done by bilateral tubal ligation. With tubal ligation the ovum does not pass to the uterus so the sperm cells cannot reach it.

Diseases caused by abnormal number of sex chromosomes are called sex aneuploidies. The main sex aneuploidies are: 44 + XXX, or trisomy X (women whose cells have an additional X chromosome); 44 + XXY, or Klinefelter's syndrome (men whose cells have an extra X chromosome); 44 + XYY, or double Y syndrome (men whose cells have an additional Y chromosome); 44 + X, Turner’s syndrome (women whose cells lack an X chromosome).

Arthropods are triploblastic (they have three germ layers) and coelomate beings.

The epithelial tissues are classified according to the shape of the cells that form it (epithelial cells may be cuboidal, columnar, or squamous) and according to the number of layers in which those  cells are placed in the tissue (into simple or stratified). The main types of epithelial tissues are simple cuboidal, simple columnar, simple squamous, stratified squamous and pseudostratified columnar (resembling more than one layer but actually having only one). There are also stratified cuboidal and stratified columnar epithelia (rare).

The normal duration of the menstrual cycle is 28 days but it can vary among different women or in different cycles of the same woman.

In the calendar contraceptive method the date n-14 (n minus 14) is taken considering n the number of days of the normal menstrual cycle of the woman (generally n=28). The safety margin +3 or –3 refers to the days around n-14 that intercourse should be avoided to prevent pregnancy. (This method is not exempt from failures. A doctor must always be consulted before relying on any contraceptive method.)

Inactivation of the X chromosome is a phenomenon that occurs in women. Since women have two X chromosomes only one of them remains active and functional mixed to the chromatin while the other remains condensed and inactive. In the same woman in some cell lineages the functional X chromosome is the one from the father and in other cell lineages the functional chromosome is the X from the mother characterizing a condition known as mosaicism (related to the X chromosome). Under the microscope the inactive X  chromosome is seen as a granule generally in the periphery of the nucleus. This granule is called the Barr body.

The criterion used to classify hosts as intermediate hosts or as definitive hosts is the kind of reproduction of the parasite, sexual or asexual, within the host. The host within which the sexual reproduction stage of the parasite occurs is the definitive host. The host within which the asexual reproduction stage of the parasite occurs is the intermediate host. 

Considering their metameric feature arthropods are proximal to annelids that also have segmented bodies. In the embryonic development of some arthropods there are fusions of metameres forming structures like, for example, the cephalothorax of arachnids.

The simple cuboidal epithelium is made of a single layer of cuboidal epithelial cells. The simple columnar epithelium is made of a single layer of prismatic cells. The simple cuboidal epithelium can be found, for example, in the renal tubules and in the walls of the thyroid follicles. The simple columnar is the epithelium that covers internally the intestines, the stomach and the gallbladder, for example.

In most vertebrate species with internal fecundation the females have reproductive cycles with fertile periods. During this period the female secretes pheromones (odoriferous substances that attract the male of the species) from the skin and mucosae. The presence of the male individual and his pheromones also stimulates the release of pheromones by the female. (Many animals also use pheromones for territorial demarcation and for signal transmission between individuals about the location of dangers and food.)

Ectoparasites are parasites that explore the external surface of the host (like, for example, mites that parasite the skin). Endoparasites are parasites that live within the body of the host (like the taenias). 

Arthropods present three distinguishing features: they are metameric beings (segmented body), they have an exoskeleton made of chitin and they present articulated limbs.

The simple squamous epithelium is made of a single layer of flat (squamous) cells. The stratified squamous epithelium is made of the same type of flat cells placed in several superimposed layers. The simple squamous epithelium is found in the pulmonary alveoli. The stratified squamous epithelium can be found in the moist mucosae, like the mucosae of the mouth, esophagus and vagina, and it is the epithelium of the skin.

One method to estimate the exact ovulation day is daily control of the body temperature taken always under same conditions. At the ovulation day the body temperature often increases about 0.5 degrees centigrade.

Hemophilia is a disease characterized by impaired blood clotting and the affected person is more prone to internal and external hemorrhages.

Patients with hemophilia A have alteration in the gene that codifies the factor VIII of blood clotting, a gene located in the non-homologous portion of the X chromosome. Patients with hemophilia B present a defect of the gene that codifies the factor IX of clotting, a gene also located in the non- homologous region of the X chromosome. Thus both diseases are X- linked diseases.

Vectors of a parasite are organisms able to transport the parasite during stages of its life cycle mediating the infection of other hosts. For example, the mosquito Aedes aegypti is the vector of the dengue virus; triatomine bugs are vectors of the Trypanosoma cruzi, protozoan that causes Chagas’ disease; mice are vectors of leptospira, bacteria that cause leptospirosis.

The external carapace of arthropods is called exoskeleton. The arthropod exoskeleton is made of chitin, a nitrogen-containing polysaccharide. 

The epidermis is the outer layer of the skin made of epithelial tissue. In the epidermis there are keratin-secreting cells (keratinocytes). Keratin is an insoluble protein that impregnates the surface of the skin providing protection and impermeability. In mammals keratin also forms the hairs. The keratinized cells of the skin surface form the corneal layer. These cells die and are continuously replaced by others.

Parthenogenesis is the reproduction or formation of a new individual from the egg cell but without fecundation by the male gamete. According to the species, individuals born by parthenogenesis may be male or female, or of any sex. In bees the drone (the single male bee) is haploid and born by parthenogenesis while the females (queen and workers) are diploid.

X-linked traits are phenotypical traits conditioned by genes located in the nonhomologous (heterologous) portions of the X chromosome.

Parasites that require only one host are called monoxenous parasites. Parasites that need more than one host for their life cycle are called heteroxenous parasites. 

Due to the presence of exoskeleton the growth of an arthropod is periodical. During the growth period the animal loses the exoskeleton, grows and develops a new exoskeleton. This process is named ecdysis, or molting.

The fish epidermis is very thin and contains mucus-secreting cells. The fish skin does not present keratin. The mucus has a protective function and it also helps the sliding of the animal under water. (The fish scales originate from the dermis and not from the epidermis.) 

In amphibians there is already a slight keratinization of the skin, probably an additional adaptation to the terrestrial environment. Amphibians have smooth and wet epidermis without scales. These features facilitate their cutaneous respiration.

The IUD (intrauterine device) is a piece of plastic coated with copper that is inserted within the uterus by a doctor. Copper is then gradually released (IUD may last 5 to 10 years) and since it has a spermicidal action sperm cells are destroyed before fecundation. Besides this mechanism the movement of the IUD inside the uterus causes slight endometrial inflammation that helps to prevent nidation.

For each correspondent gene to an X- linked trait women present always two alleles since they have two X chromosomes. Men present only one allele of genes related to X-linked traits since they have one X chromosome.

An etiological agent of disease is the agent that causes the disease. It may be a living being, substance or environmental fact. 

The digestive tube of arthropods is complete, containing mouth and anus. Arthropods are protostome animals, i.e., in their embryonic development the blastopore originates the mouth.

In the epidermis of birds and reptiles there are practically no glands. In mammals there are sweat glands and sebaceous glands.

The embryonic growth depends directly on mitosis. Through this type of cell division the zygote divides itself giving birth to a series of cells that by mitosis also compose differentiated tissues and organs until the formation of a complete individual.

Hemophilia is a disease characterized by impaired blood clotting and the affected person is more prone to internal and external hemorrhages.

Patients with hemophilia A have alteration in the gene that codifies the factor VIII of blood clotting, a gene located in the non-homologous portion of the X chromosome. Patients with hemophilia B present a defect of the gene that codifies the factor IX of clotting, a gene also located in the non- homologous region of the X chromosome. Thus both diseases are X- linked diseases.

Endemic diseases are those that often affect people of a given place, many or few individuals. Epidemic diseases are those of rapid spread and elevated number of new cases. An endemic disease can turn into an epidemic disease.

Since they have exoskeleton and periodic ecdysis, the growth of arthropods is limited to avoid the animal becoming vulnerable to environmental harm. There are however some arthropod species with relatively large- sized individuals, like “giant” cockroaches, crabs and spiders.

The formation of complex and distinct pluricellular organisms would not be possible if mitosis in embryos produced only daughter cells with an identical life history as the mother cell, since there would not be differentiation and structural or functional specialization among cells.

Evolution solved the problem creating the cellular differentiation process by which, motivated by stimulus not yet well-known by science, different and specialized cell lineages gave birth to different tissues, organs and systems that, as a whole, form the pluricellular organisms.

Cellular differentiation probably is a very intricate process that activates and inactivates some genes within the cell in response to some stimulus.

Melanocytes are epithelial cells of the skin specialized in secretion of melanin. Melanin is a pigment that besides coloring the skin, the iris of the eye and the hair, also works as a filter against the ultraviolet radiation of the sun thus protecting the body against the harmful effects of this radiation (mainly burns and carcinogenic mutations). Melanocytes are the cells affected in one of the more deadly skin cancers: melanoma.

The four initial stages of the embryonic development are the morula stage, the blastula stage, the gastrula stage and the neurula stage.

Considering the alleles Xh and X, where Xh represents the allele that conditions hemophilia A, in women the possible genotypes are XX, XXh and XhXh. In men the possible genotypes are XY and XhY. Concerning the phenotypes, factor VIII is produced in every individual with at least one nonaffected X chromosome. So the women XX and X Xh and the men XY are normal. Only women XhXh and men XhY have the disease.

The main human bacterial infections transmitted by respiratory secretions (sneezes, cough) and saliva drops are: bacterial pneumonias, tuberculosis, whooping cough (pertussis), diphtheria, bacterial meningitis. Main bacterial diseases transmitted by blood or sexual contact are: gonorrhea, syphilis. Main bacterial diseases transmitted by animal vectors are: bubonic plague, endemic typhus, leptospirosis. Some bacterial diseases transmitted through fecal-oral route and contaminated food are: cholera, typhoid fever. Other important bacterial infections: Hansen's disease, possibly transmitted by saliva drops and contact with injured skin and mucosae; trachoma, eye disease transmitted by ocular secretions; tetanus, transmitted when the etiological agent enters the body through skin wounds. 

Arachnids can inoculate poison to paralyze or kill their preys using structures called chelicerae. The prey is partially digested outside the body of the arachnid by digestive enzymes inoculated together with the venom or injected posteriorly. After this extracorporeal digestion the food is ingested and gains the digestive tube of the predator where the extracellular digestion continues.

The main animal cell tissues are the epithelial tissue, the nervous tissue, the muscle tissue and the connective tissue.

The main organs and tissues that are part of the musculoskeletal system in humans are the cartilages, the bones and the muscles.

The cell division in the first stage of the embryonic developments is called cleavage, or segmentation. In this stage several mitoses occur from the zygote forming the new embryo.

There are more hemophilic men than hemophilic women because women need to have two X chromosomes affected to develop the disease while in men the disease manifests when the single X chromosome is affected.

The vaccine against tuberculosis is called BCG (bacillus Calmette-Guérin). BCG is not used in some countries where tuberculosis is not so prevalent because it can distort later diagnostic studies of the disease; in other countries, like Brazil, it is obligatory for children. The vaccine is made of attenuated TB bacteria. 

In the arthropod exoskeleton there is a layer of wax which is impermeable. This feature was fundamental for primitive arthropods from the sea to survive on dry land without losing excessive water to the environment.

Epithelial tissues, also called epithelia, are tissues specialized in the covering of external and internal surfaces of the body.

The general function of the epithelium is to provide protection and impermeability (or selective permeability) to the covered structure. This justifies the epithelium's typical features: the cellular juxtaposition forming layers of very proximate cells with diminished or none intercellular space between each two neighbor cells.

The main cells of the cartilages are the chondrocytes, originated from the chondroblasts that secrete the intersticial matrix. There are also chondroclasts, cells with many lisosomes and responsible for the digestion and remodelation of the cartilaginous matrix.

The cells that result from the cleavage (the first stage of the embryonic development) are called blastomeres. In this stage the embryo is called morula (similar to a “morus”, mulberry).

Daltonism is a recessive X-linked inheritance (gene situated in the nonhomologous portion of the X chromosome). Daltonism is more frequent in men since in them only the single X chromosome needs to be affected for the disease to manifest. In women it is necessary for both X chromosomes to be affected for the disease to come out. The disease appears due to a defect in the gene that codifies a retinal pigment sensitive to red.

In crustaceans, typical aquatic beings, there are richly vascularized gills that make contact with water and permit gas exchange. In terrestrial insects the respiration is tracheal and gases flow inside small tubes that connect the animal external surface and ramify to tissues and cells without the participation of blood. In arachnids, besides the tracheal respiration, book lungs (thin folds resembling leaves in a book) may also exist.

The nervous tissue is formed of neurons and glial cells. The function of the nervous tissue is to receive and to transmit neural impulses (reception and transmission of information). This function justifies the characteristic morphology of neurons, with membrane projections (dendrites) to get information and an elongated membrane projection (axon, or nerve fiber) to transmit information at distance. In their turn, the glial cells support the neurons and facilitate their work (sometimes acting as insulators).

The musculoskeletal system has the functions of supporting and protecting organs, maintenance of the body spatial conformation, motion of organs, limbs and bodily portions and nutrient storage (glycogen in muscles, calcium and phosphorus in bones).

The animal pole of a telolecithal egg is the portion of the egg with little vitellus, it is opposite to the vegetal pole which is the region where the yolk is concentrated.

If mothers are not affected by the disease and noncarriers of the gene (do not have an Xh allele) it is impossible for their sons to be hemophilic since the X chromosome of males always comes from the mother. Hemophilic sons are only possible when the mother is hemophilic (homozygous for the hemophilic gene, a very rare situation) or carriers of an affected X chromosome (XXh).

Pneumonia is the generic name of inflammation of the lungs. Besides bacterial pneumonias, there are pneumonias caused by virus, fungi, toxic pneumonias, etc. 

Respiratory pigments are molecules able to carry oxygen and other respiratory gases present in circulatory fluids.

In crustaceans and in arachnids hemocyanin is the respiratory pigment. In humans the analogous pigment is hemoglobin.

Muscle tissues are tissues made of cells able to perform contractions and thus to generate movement.

The function of the muscle tissue is to pull bones (skeletal striated muscle), to contract and move viscera and vessels (smooth muscle) and to make the heart to beat (cardiac striated muscle). The muscle cells have internal structures called sarcomeres where there are myosin and actin molecules disposed to create contraction and distension (movement).

The cartilaginous matrix is made of collagen fibers, mainly collagen type II, and of proteoglycans, proteins associated to glycosaminoglycans, chiefly hyaluronic acid. The proteoglycans provide the typical rigidity of the cartilages.

Vitellus (yolk) is the nutritive material that accumulates in the cytoplasm of the egg (zygote) with the function of nourishing the embryo. According to the amount of vitellus in them, the vertebrate eggs are classified as oligolecithal (little yolk), centrolecithal, or heterolecithal (more yolk diffusely distributed) and telolecithal (more yolk concentrated in one end of the egg).

There are many X-linked diseases, like hemophilia A, hemophilia B and adrenoleukodystrophy, but known Y- linked diseases are few and very rare.

Cholera is a bacterial disease caused by the Vibrio cholerae. The disease is transmitted by fecal-oral route and the main mode of transmission is ingestion of contaminated water or food. It is most prevalent in places that lack adequate sanitary conditions.

Inside the human gut the cholera vibrion releases toxins called enterotoxins. The infection can cause intense diarrhea, vomiting, dehydration and even death in more severe cases. 

In arthropods the impermeability of the exoskeleton makes the passage of gases difficult. In addition the new methods of respiration present in arthropods were preserved by evolution because they were more efficient for those animals.

The typical function of the connective tissues is to fill empty spaces among other body tissues.

This function is related to the great capability of the cells of the connective tissue to secrete substances that constitute extracellular material, like collagen and elastic fibers, creating a significant spacing between these cells.

(There are other important biological features of the connective tissues, such as substance transportation, defense of the organism, etc.)

The cartilaginous and the osseous tissues are considered connective tissues since they are tissues in which the cells are relatively distant from others with a great amount of extracellular matrix in the interstitial space.

After passing the morula stage in which the embryo is a compact mass of cells, the next stage is the blastula stage. In the blastula stage the compactness is lost and an internal cavity filled with fluid appears inside, the blastocele.

The X-linked daltonism is a disease in which the affected individual sees the red color as green or confounds these two colours.

Meningitis is the generic name given to inflammation of the meninges, membranes that cover the central nervous system. Meningitis can have several causes (infectious, toxic, traumatic, neoplastic infestation, autoimmune). Bacterial infections caused by meningococcus, haemophilus, pneumococcus or by tuberculosis bacteria are severe and contagious.

The main symptoms of bacterial meningitis are high fever, nuchal rigidity, intense headache, vomiting and sometimes convulsions. The disease should be treated with antibiotics. 

In arthropods the respiratory system is open (lacunar). Blood, also known as hemolymph, is pumped by a heart and falls into cavities (lacunas) irrigating and draining tissues.

All arthropods have a heart. Crustaceans and arachnids have respiratory pigments. Insects do not have respiratory pigments since their blood does not carry gases (in them gases reach tissues and cells through tracheal structures).

Bones and cartilages, tissues with great amount of intercellular material, are formed of connective tissue.

Cartilages are responsible for the structural support of the nose and ears. The trachea and the bronchi are also organs with cartilaginous structures that prevent the closing of these tubes. In joints there are cartilages that cover the bones providing a smooth surface to reduce the friction of the joint movement. In the formation of bones the cartilages act as a mold and they are gradually substituted by the osseous tissue.

Archenteron is the tube formed during gastrulation by means of invagination of the blastula wall inside the blatocele. It is the origin of the gastrointestinal tract. Blastopore is the opening of the archenteron to the exterior. The blastopore gives birth to one of the extremities of the digestive tube: the mouth in protostome beings, or the anus in deuterostome beings.

Holandric genes are genes situated in the nonhomologous region of the Y chromosome. Holandric genes condition phenotypes that emerge only in men since individuals of the female sex do not present in their X chromosomes genes from the nonhomologous portion of the Y chromosome (existent only in men). A widely known holandric gene is the one that conditions hypertrichosis pinnae (hair in the ears), a phenotype inherited from fathers to sons through the Y chromosome.

The etiological agent of Hansen’s disease is bacteria called Mycobacterium leprae. The mode of transmission is not yet totally known but it is believed that respiratory secretions and saliva drops can spread the disease. Hansen’s disease is a chronic disease (slow progression) that generally attacks the skin and the peripheral nerves although other areas of the body can be affected. In the skin nodules, reddish spots, thickening of the dermis and lack of sensitivity appear; the mucosae, especially the nasal mucosa, may be injured and also the viscera may be affected. The main form of prevention is information, since there is available treatment; infected people should, as soon as possible, look for health services for evaluation and treatment of the disease. In the past Hansen’s disease was called leprosy. 

Even having low speed and low pressure circulatory system, since it is a lacunar (open) circulatory system, insects perform extremely fast and exhaustive movements with their muscle fibers, like wing beating. This is possible because in these animals the respiration is independent from the open circulation. Gas exchange is done with great speed and efficiency by the tracheal system that puts cells in direct contact with air. Muscles can then work fast and hard.

The relative great spacing between cells is a typical feature of the connective tissue. There are much intercellular material generally secreted by the tissue cells.

The Haversian canals are longitudinal canals present in the osseous tissue within which blood vessels and nerves pass. The osseous tissue distributes itself in a concentric manner around these canals. The Volkmann’s canals are communications between the Harvesian canals. The osseous tissue is highly vascularized in its interior. 

The blastula turns into gastrula in a process known as gastrulation.

Sex-influenced dominance is the phenomenon in which the manifestation of a phenotype of a gene in heterozygosity depends on the sex of the individual. For example, hereditary baldness is a dominant phenotypical form if the individual is male and it is a recessive form if the individual is female.

In crustaceans a pair of excretory organs called green glands exists. The green glands collect residuals from the blood and other parts of the body. They are connected by ducts to excretory pores located under the base of the antennae and these pores release the excretions outside.

In insects small structures called malpighian tubules gather wastes from the blood and throw them into excretory ducts that open in the intestine. In these animals excretions are eliminated together with feces.

In arachnids, besides malpighian tubules, there are coxal glands located in the cephalothorax near the limbs that also participate in excretion.

The main functions of the connective tissues are: supporting and filling of spaces; cellular nutrition; energetic storage (fats); hematopoiesis (formation of blood, blood cells and blood components); immune defense (specialized cells).

The three main cell types of the osseous tissue are the osteoblasts, the osteocytes and the osteoclasts. Osteoblasts are known as bone-forming cells since they are the cells that secrete the proteinaceous part of the bone matrix (collagen, glycoproteins and proteoglycans). The bone matrix is the intercellular space where the mineral substances of the bones are deposited. Osteocytes are differentiated mature osteoblasts formed after these cells are completely surrounded by the bone matrix. Osteocytes have the function of supporting the tissue. Osteoclasts are the giant multinucleate cells that remodelate the osseous tissue. They are originated from monocytes and they contain many lisosomes. Osteoblasts secrete enzymes that digest the osseous matrix creating canals throughout the tissue.

The mesoderm appears from differentiation of endodermal cells that cover the dorsal region of the archenteron.

In humans the main blood group systems are the ABO system, the Rh system and the MN system.

Antibiogram is a laboratory test intended to guide the choice of adequate antibiotic to treat a given bacterial infection. In the antibiogram cultures of bacteria obtained from tissues contaminated by the infection under study are submitted to the action of different antibiotics. After some time it is verified which of the antibiotics were successful in interrupting the bacterial growth or in killing the bacterial population.

The antibiogram is very important to avoid exaggerated and inefficient use of antibiotics and the emergence of multiresistant bacteria.

Arthropods have compound eyes made of several visual units called ommatidia. Each ommatidium transmits visual information through the optic nerve to the brain, which interprets the image. Because they are round and numerous, these ommatidia, whose external surfaces point in different directions creating independent images, cause arthropod eyes have a large visual field, larger than the visual field of vertebrates. Some insects have one or more simple eye besides their pair of compound eyes.

The matrix of the connective tissue proper is made of collagen fibers, elastic fibers and reticular fibers.

Bone matrix is the content that fills the intercellular space of the osseous tissue. The bone matrix is made of mineral substances (about 5%), mainly phosphorus and calcium salts, and organic substances (95%), mainly collagen, glycoproteins and proteoglycans.

The three germ layers are the ectoderm, the mesoderm and the endoderm.

The blood types of the ABO blood system are the type A, the type B, the type AB and the type O.

Those diseases are caused by the protozoans, beings of the kingdom Protista.

Reproduction in beings of the phylum Arthropoda is sexual, with larval stage in some insects and crustaceans (arachnids present only direct development).

The name connective tissue proper is used to designate the connective tissue that fills interstitial spaces as opposed to the specialized connective tissues (blood, bones, cartilage, adipose tissue, etc.). The connective tissue proper secretes collagen, elastin and reticular fibers.

The main functions of the osseous tissue are: to provide structural rigidity to the body and to delineate the spatial positioning of the other tissues and organs; to support the body weight; to serve as a site for mineral storage, mainly of calcium and phosphorus; to form protective structures for important organs like the brain, the spinal cord, the heart and the lungs; to work as a lever and support for the muscles, providing movement; to contain the bone marrow where hematopoiesis occurs.

Gastrulation is the process through which a portion of the blastula wall undergoes invagination inside the blastocele, forming a tube called archenteron (primitive intestine). The cells of the inner side of the tube form the endoderm (germ layer) and the cells of the outer side form the ectoderm (another germ layer). It is the beginning of the tissue differentiation in embryonic development.

Red blood cells have different antigens in the outer surface of their plasma membrane; for example, the antigens A and B of the ABO system are glycoproteins of the membrane. If a donor has red blood cells with antigens not present in the red blood cells of the recipient (lacking of transfusion compatibility) the immune system of the recipient recognizes these molecules as actual antigens (i.e., foreign substances) and triggers a defense response producing specific antibodies against those antigens. The transfused red blood cells then are destroyed by these antibodies and the recipient individual may even die.

The etiological agent of Chagas’ disease is the Trypanosoma cruzi. The name “cruzi” was given in honor of the Brazilian doctor Oswaldo Cruz. The disease was named after the Brazilian doctor Carlos Chagas.

In arthropods the nervous system has more sophisticated sensory receptors with well- advanced cephalization. In the anterior region of the body there is a fusion of ganglia forming a brain connected to two ventral ganglial chains having motor and sensory nerves.

The boosted development of the sensory system of arthropods provides more adaptive possibilities for these animals to explore many different environments.

There are different collagen types. The main function of these proteins is to keep the shape and the structural rigidity of the tissue. (Collagen is the most abundant protein of the human body.)

The main bones of the body may be classified as flat or long bones (there are bones not classified into these categories). Examples of flat bones are the skull, the ribs, the hipbones, the scapulae and the sternum. Examples of long bones are the humerus, the radius, the ulna, the femur, the tibia and the fibula.

Cnidarians are diploblastic, i.e., they present only endoderm and ectoderm. With the exception of poriferans, all remaining animals are triploblastic. Poriferans do not present differentiated tissue organization and so they do not classify regarding germ layers (although sometimes they are mentioned as diploblastic).

Type A: antigen A, antibody anti-B. 

Type B: antigen B, antibody anti-A.

 Type AB: antigens A and B, does not produce antibody A neither antibody B. 

Type O: does not have antigen A neither antigen B, has antibodies anti-A and anti-B.

(Obviously antibodies are made by B lymphocytes not by red blood cells.)

In the definitive hosts as well as in triatomine bugs (intermediate hosts) the protozoan that causes Chagas’ disease alternates mastigote (flagellate) and amastigote forms and also intermediate forms between these forms. 

In arthropods there are species having external fecundation and other species having internal fecundation. Internal fecundation is predominant.

The main cells of the connective tissue proper are the fibroblasts, cells that secrete the intercellular material. These cells are the majority of cells of the tissue. Fibroblasts later are transformed into fibrocytes, mature cells with restricted secretory role.

The intercellular substance that fills the interstice is called interstitial matrix, or just matrix.

The myocardium of the heart is made of cardiac striated muscle tissue.

Coeloms are cavities delimited by mesoderm. Coeloms originate the cavities where the internal organs of the body are located, like the pericardial cavity, the peritoneal cavity and the pleural cavity. Besides coelomate animals, there are acoelomate animals, like platyhelminthes, and pseudocoelomate animals, like nematodes.

The ABO blood system includes the erythrocytic antigens A and B that can be attacked by the antibodies anti-A and anti-B. The antigens A and B are agglutinogens and the antibodies anti-A and anti-B are agglutinins.

The vector of Chagas’ disease is its intermediate host, a triatomine bug. The main species is Triatoma infestans. Hemipteran insects, like triatiomines, have sucking mouthparts that can be used to suck blood from animals or organic fluids from plants. The vectors of Chagas’ disease are hematophagous hemipterans that have nocturnal habits. The blood-sucking bugs become infected when they bite a contaminated person. The parasites then multiply within the bug gut and are eliminated with its feces. When a contaminated triatomine bites another person it defecates near the bite site and the released protozoans can penetrate into the definitive host through mucosae or through the bite wound. Wild and domestic mammals can also be vessels for the disease. 

Fecundation in insects is internal, with copulation.

The elastic fibers are made of a protein called elastin. Elastic fibers abound in artery walls, helping the maintenance of the arterial blood pressure in these vessels. They are also present in the lungs, providing them with elasticity (some respiratory diseases are caused by destruction of these fibers). In many other organs and tissues the elastic fibers are found in the interstitial matrix.

The smooth muscle tissue is responsible for the peristaltic movements of the intestines. The smooth muscles are not controlled by volition.

The coeloms are originated from mesoderm.

The transfusional compatibility for the ABO system takes into account the antigens present in the red blood cells of the donor and the antibodies that the recipient can produce. Whenever the recipient is not able to produce antibodies against antigens of the red blood cells of the donor the transfusion is compatible.

So regarding ABO compatibility type A can donate to type A and to type AB. Type B can donate to type B and to type AB. Type AB can donate only to type AB. Type O can donate to all ABO types.

(Blood transfusion must be studied, planned and supervised by doctors.)

Trypanosoma cruzi is a heteroxenous parasite, i.e., it has an intermediate host, the triatomine bug, and a definitive host, the human. The triatomine bug becomes infected by sucking the blood of a contaminated person. Within the bug gut the protozoan reproduces itself. When the triatomine bites another person it defecates near the bite site. Generally the bitten person itches the area of the bite and the parasite gains the circulation of the definitive host. Within humans the Trypanosoma cruzi multiply as amastigote form in the cardiac muscle tissue or in the nervous tissue forming pseudocysts. These pseudocysts break releasing flagellate parasites into the circulation and the cycle is repeated.

In crustaceans there are species with direct and others with indirect development. In insects there are species without larval stage (ametabolic insects), others undergoing indirect development beginning with an egg stage followed by a nymph stage (hemimetabolic insects) and others with indirect development beginning with the larval stage (holometabolic insects).

The transformation of a larva into an adult individual is called metamorphosis. Hemimetabolic insects undergo incomplete metamorphosis while holometabolic insects undergo complete metamorphosis.

The reticular fibers are very delicate interstitial fibers made of a special type of collagen known as collagen type III. They can be found in many organs and tissues such as in lymphnodes, in the spleen, in the liver, in blood vessels and also covering muscle fibers.

There are three types of muscle tissue: the skeletal striated muscle tissue, the cardiac striated muscle tissue and the smooth muscle tissue. The striated muscles present under microscopic view transversal stripes and their fibers (cells) are multinucleate (in the skeletal) or may have more than one nucleus (in the cardiac). The smooth muscle does not present transversal stripes and it has spindle- shaped fibers each with only one nucleus.

The neurula stage is characterized by the appearance of the neural tube along the dorsal region of the embryo. The growth of mesoderm in that region induces the differentiation of ectodermal cells just above. These cells then differentiate forming the neural tube. So the origin of the nervous system is the ectoderm (the same germ layer that gives birth to the skin).

The inheritance of the ABO blood system is a multiple alleles inheritance. There are three involved alleles, I_A, I_B and i that combine in pairs to form the genotypes.

Concerning dominance, the allele i is recessive in relation to the alleles I_A and I_B. Between I_A and I_B however lack of dominance is established with the heterozygous (I_AI_B) manifesting distinct phenotype.

Incubation period is the time interval between the infection by an agent that causes disease and the first signs or symptoms of the disease.

Nymphs are larvae of hemimetabolic insects (like grasshoppers). They are very similar to the adult insect although smaller. In holometabolic insects (like butterflies) the larva makes a cocoon (chrysalis, pupa) where it lives until emerging into the adult form. Imago is the name given to the adult form of insects with indirect development.

Diseases of the connective tissue are hereditary or acquired diseases(many of autoimmune cause) characterized by deficiency in structure or function of components of the connective tissue, for example, deficiencies of collagen, elastin, etc. Some of such diseases are lupus, dermatomyositis, cheloid, scleroderma, mixed connective tissue disease, mucinosis and Marfan's syndrome.

The bones are moved by the skeletal striated muscles. These muscles are voluntary (controlled by volition).

Pleura is the membrane that covers the lungs and the inner wall of the chest; pericardium is the membrane that covers the heart; peritoneum is the membrane that covers most organs of the gastrointestinal tract and part of the abdominal cavity. All these membranes delimit coeloms (internal cavities).

Since the alleles are I_A, I_B and i the possible genotypes are I_AI_A (blood type A),  I_AI_B (blood type AB), I_BI_B (blood type B) and ii (blood type O).

The period when the larva is within its cocoon is a time of intense biological activity since the larva is being transformed into an adult animal.

The blood is a means of substance transportation throughout the body. The blood distributes nutrients, oxygen, hormones, antibodies and cells specialized in defense to the tissues and collects residuals like nitrogen wastes and carbon dioxide from them.

The esophageal wall in its superior portion is made of skeletal striated muscle. The inferior portion is made of smooth muscle. In the intermediate portion there are skeletal striated and smooth muscles. All of these muscles are important to push the food down towards the stomach.

In a schematic longitudinal section of the embryo after the neurula stage, the outermost layer of cells is the ectoderm. In the ventral region comes the archenteron tube formed of endodermal cells. In both sides of the embryo coeloms delimited by mesoderm are present. In the central region above the archenteron and in the middle of the coeloms there is the notochord. In the dorsal region just above the notochord lies the neural tube.

Universals donors of the ABO blood type system are the individuals of the type O. Type O blood does not have antigen A neither antigen B in its red blood cells and can be donated to individuals of any ABO type Universal recipients of the ABO blood type system are the individuals of the type AB. Type AB blood does not contain antibody anti-A neither antibody anti-B and people of this group can receive blood from any of the ABO types.

Chagas’ disease may or may not present an acute phase. When it is present, the incubation period is about 5 to 14 days. The chronic phase, however, can manifest in more than 10 years after the infection.

At the site where the trypanosoma has penetrated the skin or the mucosa becomes swelled: This sign is known as chagoma. Another sign that may appear in up to 20% of cases after the infection is the Romana’s sign, a swelling of one of the eyelids when infection took place through the ocular route. In the acute Chagas’ disease fever, malaise, inflammation and enlargement of lymph nodes may occur. In more severe cases fatal inflammation of the cardiac muscle or of the meninges may happen.

Crustaceans and arachnids do not have wings. Most insects have wings.

The blood is made of a liquid and a cellular part. The fluid part is called plasma and in it there are several substances, like proteins, lipids, carbohydrates and mineral salts. The cellular constituents of the blood are also known as blood corpuscles and they comprise the erythrocytes (red blood cells), leukocytes and platelets.

The functional units of the muscle fibers are the sarcomeres. Within the sarcomeres blocks of actin and myosin molecules are posed in organized manner. The sarcomeres align in sequence forming myofibrils that are longitudinally placed in the cytoplasm of the muscle fibers (cells). The grouping of consecutive blocks of actin and myosin in parallel filaments creates the striped pattern of the striated muscle tissue seen under the microscope.

Epidermis and nervous system have the same embryonic origin: the ectoderm. The epidermal appendages (like nails, hair, sweat glands and sebaceous glands), the mammary glands, the adenohypophysis, the cornea, the crystalline lens and the retina are also derived from ectoderm.

By using the ABO blood typing it is possible only to exclude paternity, maternity or brotherhood/sisterhood but it is not possible to conclude positively about these relationships. 

For example, if an individual has type O blood, ii genotype, he or she cannot have biological parents of the type AB (I_AI_B genotype) since necessarily one of his/her alleles has come from the father and the other from the mother. Another example: a couple of individuals of the type O (ii) in their turn can only generate direct offspring of the type O blood, since they do not have alleles that condition antigen A neither antigen B.

Prophylaxis are measures taken to prevent diseases. For example, the use of condoms in sexual relations is a prophylaxis against contamination by agents that cause STDs (sexually transmitted diseases).

Besides the phylum Arthropoda another animal phylum with flying creatures is the chordate phylum, birds and chiropterans mammals (bats) have wings. In the past some reptiles that possibly originated the aves had wings too. There are also amphibians and fishes that jump high exploring the aerial environment.

Hematopoiesis is the formation of blood cells and other constituent elements of the blood.

Sarcomeres are the contractile units of the muscle tissue formed of alternating actin blocks (thin filaments) and myosin blocks (thick filaments). Several sarcomeres placed in linear sequence form a myofibril. Therefore one muscle fiber (cell) has many myofibrils made of sacomeres. The compartments where myofibrils are inserted are delimited by an excitable membrane known as sarcolemma. The sarcolemma is the plasma membrane of the muscle cell.

Somites are differentiated portions of mesodermal tissue longitudinally distributed along the embryo. The somites originate the muscle tissue and portions of the connective tissues.

Anti-Rh antibodies are made by humoral immune response. When an Rh- individual makes contact with the Rh factor this is recognized as foreign (antigen), the primary immune response begins and small amounts of anti-Rh antibodies and memory B lymphocytes are made. In future contact with the antigen there will already be circulating antibodies and memory immune cells prepared to create an intense and effective attack against the Rh factor.

The vector of malaria is a mosquito of the genus Anopheles, also called anopheline. In opposition to the mosquito vector of the dengue fever, the anopheline has nocturnal habits. 

Sponges live exclusively in an aquatic environment and they are attached by their base to a substrate (fixation ground). Sponges are filtering animals, they nourish themselves from nutrients that enter their atrium brought in with water.

Hematopoiesis occurs in the bone marrow (mainly within flat bones), where erythrocytes, leukocytes and platelets are made, and in the lymphoid tissue, responsible for the maturation of leukocytes and found in the thymus, spleen and lymphnodes.

Schematically actin filaments attached perpendicularly to both sarcomere extremities (longitudinal sides) make contact with myosin filaments positioned in the middle of the sarcomere and in parallel to the actin filaments. Before the contraction the sarcomeres are extended (relaxed) since the contact between actin and myosin filaments is only made by their extremities. During contraction actin filaments slide along the myosin filaments and the sarcomeres shorten.

Blood cells have a mesodermal embryonic origin. Other organs made from mesoderm are: covering serous membranes like the pericardium, the peritoneum and the pleura, muscles, cartilages, dermis, adipose tissue, kidneys, ureters, bladder, urethra, gonads, blood and lymph vessels, bones.

Besides ABO blood compatibility the compatibility concerning the Rh blood system must also be checked. In addition it is of fundamental importance for the safety of blood transfusion performing tests to detect agents of main blood transmitted infectious diseases, like HIV (AIDS), hepatitis B and C, syphilis, Chagas disease, etc.

(Any transfusion must be studied, planned and supervised by doctors.)

The etiological agents of malaria are protozoans of the genus Plasmodium. There are four different types of plasmodia that cause malaria: Plasmodium malariae, Plasmodium vivax, Plasmodium falciparum and Plasmodium ovale. 

Sponges can close their pores to avoid the entrance of water into their bodies in the presence of stimulus that may mean danger. This method however is rudimentary but it is actually a protection attempt against nocent agents.

Bone marrow can mainly be found in the internal cavities of flat bones, like the vertebrae, the ribs, the scapulae, the sternum and the hips.

The bone marrow is not made of osseous tissue, although it is a connective tissue as bone tissue is.

In the sarcomere there are organized actin and myosin blocks. Troponin and tropomyosin also appear associated to actin. The actin molecules when activated by calcium ions liberated in the proximities of the sarcomere are pulled by myosin molecules. This interaction between action and myosin shortens the myofibrils originating the phenomenon of muscle contraction.

Histogenesis is the process of tissue formation in the embryonic development. Organogenesis is the process of organ formation. Before histogenesis and organogenesis the primitive embryonic structures have been already formed: germ layers, neural tube, notochord, coeloms, somites.

Blood typing is the determination, by means of tests, of the classification of a blood sample concerning blood group systems (specially the ABO system and the Rh system).

In the life cycle of the plasmodium humans are the intermediate hosts (where asexual reproduction takes place) and the vector mosquito is the definitive host (where sexual reproduction occurs). 

Sponges have bodies in the form of tubular vases or globes open in the upper extremity. They have an internal central cavity and porous walls. The central cavity is called spongocoel and the opening in the upper extremity is called osculum.

Stem cells are undifferentiated cells able to differentiate into other types of specialized cells.

The stem cells of the bone marrow originate the differentiated blood cells. According to stimulus from specific growth factors the stem cells are turned into red blood cells, leukocytes and megakaryocytes (cells that form platelets). Research shows that stem cells of the bone marrow can also differentiate into muscle, nervous and hepatic cells.

The liver and the pancreas are originated from the endoderm. Also from endodermal origin are the epithelia of the airway, the epithelia of the bladder, of the urethra and of the GI tube (except of the mouth and anus), the alveolar cells of the lungs and the thyroid and parathyroid glands.

RH factor is a protein of the red blood cell plasma membrane that behaves as antigen in blood transfusions triggering a humoral (antibody-based) immune response. Most people present the protein in their red blood cells and are part of the Rh+ group. People that do not have the protein classify as Rh-. 

The origin of the name Rh factor is related to the first researches that discovered this blood antigen was in rhesus monkeys (“Macaca mulatta”).

The plasmodium infects the human blood causing destruction of red blood cells and it also affects the liver. Malaria characterizes by periodical episodes of fever, chills and sweating that can be accompanied by headache, nausea, vomiting and jaundice. The destruction of red blood cells may lead to anemia and hypoxemia.

The infection by Plasmodium falciparum if not treated can cause other complications and even death. 

Apart from sponges, that do not have a digestive cavity where extracellular digestion takes place, all other animals have a digestive system with an internal cavity in which extracellular digestion occurs.

Erythrocytes are also known as red blood cells (RBCs), or red corpuscles. Red blood cells are responsible for oxygen transport from the lungs to the tissues.

If oxygen from hemoglobin or myoglobin is not enough for the energy supply of the muscle cell the cell then begins to do lactic fermentation in an attempt to compensate the deficiency. The lactic fermentation releases lactic acid and this substance causes muscle fatigue and predisposes the muscles to cramps.

Twins are simultaneously generated (within the mother’s uterus) offspring. Twins classify according to zygosity as monozygotic or as dizygotic twins. Monozygotic twins, also known as identical twins, are those originated from one single fertilized ovum (therefore from one single zygote); monozygotic twins are genetically identical, i.e., they have identical genotypes and are necessarily of the same sex. Dizygotic twins, also known as fraternal twins, are those generated from two different ova fecundated by two different sperm cells; so they are not genetically identical and they are not necessarily of the same sex.

The vector mosquito bites a contaminated person and ingests female and male gametocytes of the parasite. Within the insect gut the gametocytes differentiate into gametes and fecundation occurs, forming zygotes. Each plasmodium zygote by mitosis (sporogony) generates numerous infective sporozoites that migrate to the salivary glands of the mosquito. When the mosquito bites a person the sporozoites enter the human circulation and when in the liver they undergo the first asexual reproduction (tissue schizogony), releasing several merozoites into the blood. The merozoites infect red blood cells where the second asexual reproduction of the cycle (erythrocytic schizogony) and the production of many other parasites occur; some of these parasites differentiate into gametocytes. The red blood cells then break (hemolysis), the parasites are released in the blood and the cycle can restart.

Crustaceans have two pairs of antennae; insects have one pair; arachnids do not have antennae.

On average red blood cells live around 120 days. The spleen is the main organ where old red blood cells are destroyed.

During the red blood cell destruction the heme groups turn into bilirubin and this substance is then captured by the liver and later excreted in the bowels as part of the bile.

Myoglobin is a pigment similar to hemoglobin and present in muscle fibers. Myoglobin has a great affinity for oxygen. It keeps oxygen bound and releases the gas under strenuous muscle work. So myoglobin acts as an oxygen reserve for the muscle cell.

Polyembryony is the phenomenon in which a single embryo in its initial embryonic stage divides itself forming many new individuals of the same sex and genetically identical. This is the way, for example, in which reproduction takes place in armadillos of the genus Dasypus. Polyembryony is an example of natural “cloning”.

In the hemolytic disease of the newborn the mother is Rh- and the fetus Rh+. In this disease antibodies produced by the mother attack the fetal red blood cells. The hemolytic disease of the new born is also known as Terythroblastosis fetalis.

The typical chills and fever episodes of malaria correspond to the phase when red blood cells are destroyed after the erythrocytic schizogony of the plasmodium life cycle. 

In crustaceans and arachnids the head is fused with the thorax forming the cephalothorax. Their body thus is divided into cephalothorax and abdomen.

In insects there are head, thorax and abdomen.

The respiratory pigment of the red blood cells is hemoglobin.

Phosphocreatine is the main means of energy storage of the muscle cells. During relaxed periods ATP molecules made by the aerobic cellular respiration transfer highly energized phosphate groups to creatine forming phosphocreatine. In exercise periods phosphocreatine and ADP resynthesize ATP to dispose energy for the muscle contraction.

Extraembryonic membranes are membranous structures that appear in parallel with the embryo and play important roles in the embryonic development. They form from the embryo but do not become part of the individual organism after its birth.

The inheritance pattern of the Rh blood system is autosomal dominant, i.e., the heterozygous manifests as Rh+. The dominance is complete (R is dominant over r). The possible genotypes are RR, Rr (both Rh+) and rr (Rh-).

Curiosity: the Rh factor is codified by a gene containing 2790 DNA nucleotides situated in the human chromosome 1.

The main preventive measures against malaria are the elimination of the vector mosquito, treatment of infected people, avoidance of the mosquito bite, information for travelers to endemic areas and the use of preventive medicines. 

Most crustaceans have five pairs of limbs. Insects have three pairs and arachnids present four pairs of limbs.

Hemoglobin is a molecule made of four polypeptide chains, each bound to a iron containing molecular group called a heme group. So the molecule contains four polypeptide chains and four heme groups.

As a protein composed of association of polypeptide chains, the functionality of hemoglobin depends upon the integrity of its quaternary structure.

The nervous cells that trigger the muscle contraction are the motor neurons. The neurotransmitter of the motor neurons is acetylcholine. When a motor neuron is excited the depolarizing current flows along the membrane of its axon until reaching the synapse at the neuromuscular junction (the neural impulse passage zone between the axon extremity and the sarcolemma). Near the axonal extremity the depolarization allows the entrance of calcium ions into the axon (note that calcium also has a relevant role here). The calcium ions stimulate the neuron to release acetylcholine in the synapse.

Acetylcholine then binds to special receptors in the outer surface of the sarcolemma, the permeability of this membrane is altered and an action potential is created. The depolarization is then conduced along the sarcolemma to the sarcoplasmic reticulum that thus releases calcium ions for the sarcomere contraction.

The yolk sac is formed from the covering of the vitellus by some cells originated from the primitive gut. The yolk sac stores vitellus, the main nourishment source of non-placental embryos.

In the hemolytic disease of the newborn the mother has Rh- blood. This mother when generating her first Rh+ child makes contact, possibly during delivery, with Rh+ red blood cells of the child and her immune system triggers the primary immune response against the Rh factor. In the next gestation in which the fetus is Rh+ the mother will already have much more anti-Rh antibodies in her circulation; these antibodies cross the placental barrier and gain the fetal circulation causing fetal hemolysis (destruction of the red blood cells of the fetus).

Some other important protozoan infections are amebiasis, giardiasis, trichomoniasis, leishmaniasis,  toxoplasmosis and meningoencephalitis by free-living amoebas.

The insects are the animal group with most diversity of species. Almost 750000 insect species are known, about 55% of the total already cataloged species of living beings (compare with mammals, with no more than 4000 known species). It is calculated however that the number of unknown species of insects may be over 2 million. The insect population on the planet is estimated to be more than 10 quintillion (1000000000000000000) individuals.

The great evolutionary success of insects is due to factors such as: small size and alimentary diversity, making possible the exploration of numerous different ecological niches; wings that provided more geographic spread; the tracheal respiration that gave them motor agility; high reproductive rates with production of great numbers of descendants.

The spleen has many functions: it participates in the destruction of old red blood cells; in it specialized leukocytes are matured; it helps the renewal of the hematopoietic tissue of the bone marrow when necessary; it can act as a spongelike organ to retain or liberate blood from or for the circulation.

Total splenectomy is not incompatible with life as none of the functions of the spleen are vital and at the same time exclusive of this organ.

Spatial summation is the recruiting of new muscle fibers to increase the muscle strength. Temporal summation occurs when a muscle fiber is continuously stimulated to contract without being able to conclude relaxation.

The permanence of a muscle fiber under a continuous state of contraction by temporal summation is known as tetany (e.g., the clinical condition of patients contaminated by the toxin of the tetanus bacteria). Tetany ends when all available energy for contraction is spent or when the stimulus ceases.

The extraembryonic membranes that may be present in vertebrates are the yolk sac, the amnion, the chorion, the allantois and the placenta.

An Rh+ donor can only donate blood to an Rh+ recipient. A person that lacks the Rh factor (Rh-) can donate to individuals of the Rh+ and Rh- groups. 

Amebiasis is caused by the protozoan Entamoeba histolytica, or simply amoeba. The transmission of the disease is oral- fecal, through contaminated water and food or by insects like cockroaches and flies. The amoeba parasites the intestine of humans generally in an asymptomatic manner, but it can sometimes cause enteritis with diarrhea and severe dysentery, abdominal pain, weight loss and anemia. In rare cases extra-intestinal invasion may occur affecting the liver, skin, genital organs and other organs. 

Insects like some species of bees, wasps, ants and termites form societies that include hierarchy and job division among members. Spiders build sophisticated external structures, webs, mainly to serve as a trap for capturing prey. Another example is the  communication mechanism in some bees known as the bee dance by which an individual signal to others information about the spatial position of flower fields and other nectar sources.

An increase in the strength of the muscle work is not achieved by increase in the intensity of the stimulation of each muscle fiber. The muscle fiber obeys an all-or-nothing rule, i.e., its contraction strength is only one and cannot be increased.

When the body needs to increase the strength of the muscle work a phenomenon known as spatial summation occurs: new muscle fibers are recruited in addition to the fibers already in action. So the strength of the muscle contraction increases only when the number of active muscle cells increases.

The allantois is the extraembryonic membrane whose function is to store the excreted matter of the embryo. In placental mammals the allantois is present but it does not exert that function since the embryonic wastes are collected by the mother’s body through the placenta.

Erythroblastosis fetalis can be prevented if in the first delivery of a Rh+ child from a Rh- mother serum containing anti-Rh antibodies is given to the mother in the first 72 hours (after the delivery). Therefore the administered anti-Rh antibodies destroy the fetal red blood cells that entered the mother’s circulation before the triggering of her primary immune response.

Giardiasis is a protozoal infection caused by Giardia lamblia, or simply giardia, a flagellate protozoan. The transmission is fecal-oral, through contaminated water and food or carried by insects like cockroaches and flies. Giardiasis manifests like amebiasis, as an enteritis with diarrhea, abdominal pain, weight loss and anemia. 

Examples of representing beings: cockroaches, flies, crabs, lobsters, shrimps, spiders, scorpions, mites. Basic morphology: segmented body (metameric), articulated limbs, chitinous exoskeleton, periodic ecdysis. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete. Respiratory system: tracheal in insects, branchial in crustaceans, tracheal and book lungs in arachnids.

Circulatory system: open, hemocyanin in crustaceans and arachnids. Excretory system: malpighian tubules in insects, green glands in crustaceans, malpighian tubules and coxal glands in arachnids.

Nervous system: ganglial. Types of reproduction: sexual, with or without larval stage in insects and crustaceans, metamorphosis in some insects, no larval stage in arachnids.

Red blood cells are erythrocytes and white blood cells are the leukocytes. Leukocytes are cells specialized in the defense of the body against strange agents and they are part of the immune system.

The main cells of the nervous system are the neurons. Besides the neurons the nervous system is also constituted of glial cells.

The presence of each extraembryonic membrane varies according to the vertebrate class. In fishes and amphibians only the yolk sac is present. In reptiles and birds besides the yolk sac there are also the amnion, the chorion and the allantois. In placental mammals besides all these membranes the placenta is present too.

The MN blood system is a third (in addition to the ABO and the Rh) system of blood antigens also related to proteins of the red blood cell plasma membrane.

The inheritance pattern of the MN blood system is autosomal with codominance, a type of lack of dominance in which the heterozygous manifests a phenotype totally distinct from the homozygous.

The possible phenotypical forms are three blood types: type M blood, type N blood and type MN blood.

Trichomoniasis is an extra-intestinal protozoan infection caused by Trichomonas vaginalis, a flagellate protozoan. The parasite infects the urinary tract of males and females. In females it causes discharge, pain and itching. Trichomoniasis is an STD because its main mode of transmission is sexual contact. 

Snails, octopuses, squids and oysters are examples of molluscs.

The types of leukocytes are lymphocytes, monocytes, neutrophils, eosinophils and basophils. Granulocytes are those in whose cytoplasm there are granules (when viewed under electronic microscopy): neutrophils, eosinophils and basophils are granulocytes. Agranulocytes are the other leukocytes: lymphocytes and monocytes. 

The integrative systems are the nervous system and the endocrine system. The designation is justified since both systems control and regulate biological functions and act at distance receiving information from organs and tissues and sending effector commands (nervous impulses or hormones) to organs and tissues thus integrating the body.

The allantois is an adaptation to dry land because in embryos of oviparous terrestrial beings, like reptiles and birds, the metabolic residuals cannot be immediately excreted to the aquatic surrounds (as fishes and amphibian larvae do). It was necessary then for the appearance of a structure capable of storing the embryonic excretes until hatching.

The name karyotype is given to the set of chromosomes of an individual, usually when visualized and identified under the microscope. The visualization generally is made with the cells in the initial phases of cell division for the chromosomes to be seen already replicated and condensed. 

There are two main forms of leishmaniasis: cutaneous leishmaniasis and visceral leishmaniasis (also known as kala-azar). The form is determined by the species of infective leishmania and by the immune response of the host. 

Molluscs can be found in the sea, freshwater and in terrestrial environments.

The generic function of leukocytes is to participate in the defense of the body against strange agents that penetrate it or are made inside the body. Leukocytosis and leukopenia are clinical conditions in which the count of leukocytes in a blood sample is abnormal. When the leukocyte count in a blood sample is above the normal level for the individual leukocytosis is defined. When the leukocyte count is lower than the expected normal level leukopenia is defined. The multiplication of these defense cells, leukocytosis, generally takes place when the body is suffering infections or in cancers of these cells. The lowering of these defense cells, or leukopenia, occurs when some diseases attack the cells, like in AIDS, or when immunosuppressor drugs are used.

In general the body creates leukocytosis as a defense reaction when it is facing infectious or pathogenic agents. The clinical condition of leukocytosis is thus a sign of infection. Leukopenia occurs when there is a deficiency in the production (for example, in bone marrow diseases) or excessive destruction of leukocytes (for example, in case of HIV infection).

Glial cells and neurons are the cells that form the nervous system. Neurons are cells that have the function of receiving and transmitting the neural impulses and glial cells (astrocytes, microgliacytes, ependymal cells and oligodendrocytes) are the cells that support, feed and insulate (electrically) the neurons. The Schwann cells that produce the myelin sheath of the peripheral nervous system can also be considered glial cells.

The chorioallantois membrane is formed by juxtaposition of some regions of the chorion and the allantois. Since it is porous, the chorioallantois membrane allows the passage of gases between the embryo and the exterior thus making aerobic cellular respiration possible.

Down syndrome is an aneuploidy, i.e., a numeric alteration of chromosomes within the cells compared to the normal number of chromosomes of the species. Affected individuals have in their cells an additional chromosome 21 instead of only one pair. For this reason the condition is also called trisomy 21. The affected person has karyotype with 47 chromosomes: 45 + XY or 45 + XX. 

Visceral leishmaniasis is caused by the protozoan Leishmania donovani. The transmission is similar to the cutaneous leishmaniasis, by the bite of sand flies. The affected organs generally are the liver, the spleen and the bone marrow. The patient often has fever, weight loss, splenomegaly (hypertrophy of the spleen), anemia and decreased counts of leukocytes and platelets. The disease may cause death.

The word “mollusc” means “soft thing”. Molluscs have soft bodies and this feature explains the name of the phylum.

The physiological mechanisms of hemorrhage contention (one of them is blood clotting) are generically named hemostasis, or hemostatic processes.

The structures that form the nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

The organs of the CNS are the brain (cerebrum, brainstem and cerebellum) and spinal cord. The PNS is made of nerves and neural ganglia. Besides these organs the meninges (dura- mater, arachnoid and pia-mater) are part of the nervous system too since they cover and protect the encephalon and the spinal cord.

Amnion is the membrane that covers the embryo. Chorion is the membrane that covers the amnion, the yolk sac and the allantois. The space delimited by the chorion and the amnion is called amniotic cavity and it is filled with aminiotic fluid. The amniotic cavity has the functions of preventing desiccation of the embryo and of protecting it against mechanical shocks.

The counting and identification of chromosomes in the karyotype of an individual can diagnose the aneuploidies, diseases caused by alteration in the number of chromosomes in relation to the normal number of the species.

The etiological agent of cutaneous leishmaniasis is the protozoan Leishmania braziliensis. The transmission, like in the visceral form of the disease, is by the bite of the sand fly Lutzomya (named after the Brazilian scientist Adolfo Lutz), the vector host. Cutaneous leshmaniasis develops in the bite site where the parasite establishes itself. The skin wound has a volcanic crater shape, a reddish injury with elevated borders. From the primary lesions the parasites can spread through the blood to affect other areas, mainly the mucosae of the nose, mouth and pharynx, causing facial deformations.

Because molluscs have a soft body they are more fragile. They also have more difficulty to support their bodies in terrestrial environments or to fixate to substrates in aquatic habitats. Many species solve these problems by secreting a calcareous carapace, or shell, an exoskeleton to support and protect them and to prevent dehydration.

Platelets, also known as thrombocytes, are fragments of giant cells of the bone marrow called megakaryocytes. With their properties of aggregation and adhesiveness they play a direct role in blood clotting and they also liberate substances that activate other hemostatic processes. Thrombocytopenia is a clinical condition in which the platelet count of the blood is lower than normal. In this situation the person becomes susceptible to hemorrhages.

The three mains parts into which a neuron can be didactically divided are: dendrites, cell body and axon.

Dendrites are projections of the plasma membrane that receive the neural impulse from other neurons. The cell body is where the nucleus and the main cellular organelles are located. Axon is the long membrane projection that transmits the neural impulse at distance to other neurons, to muscle cells and to other effector cells.

True placenta is present in placental mammals. The placenta is formed from the chorion of the embryo and from the mother’s endometrium. Its main function is to allow the exchange of substances between the fetus and the mother’s body.

Colchicine is a substance that disallows the formation of microtubules and thus of the spindle fibers in cell division. Under the action of this drug the cells interrupt division at metaphase and the anaphase does not occur. Therefore the use of colchicine in the study of karyotypes is interesting because chromosomes will be seen replicated and condensed.

In molluscs there is not a body divided into segments. Considering just this feature it could be said that evolutionarily they are more proximal to nematodes than to annelids.

When there is some tissue wound with injury of blood vessel the platelets and endothelial cells of the wall of the damaged vessel liberate substances (respectively platelet factors and tissue factors) that trigger the clotting process.

The terminal portion of the axon is called presynaptic membrane. Through this membrane neurotransmitters are released into the synaptic junction.

The amnion is also an adaptation to dry land since one of its functions is to prevent desiccation of the embryo.

Aneuploidy is an abnormal number of chromosomes in the cells of an individual. The main aneuploidies of the human species and their respective conditions are: the nullisomies (absence of any chromosome pair of the species, often incompatible with life); the monosomies (absence of a chromosome from a pair, for example, Turner’s syndrome, 44 + X); the trisomies (an extra chromosome, for example, the triple X syndrome, 44 + XXX, or the Edwards syndrome, trisomy 18, 45 + XY or 45 + XX).

Bat and pigeon feces can carry Histoplasma capsulatum, the fungus agent of histoplasmosis. The infection is transmitted through inhalation of contaminated dust in places visited by these animals (caves, tunnels, squares, roofs, etc.). Cryptococcosis is another fungal disease transmitted by pigeon excrement. 

The phylum Mollusca is divided into five main classes: pelecypods, or bivalves (Pelecypoda, or Bivalvia), includes oysters, clams, mussels; gastropods (Gastropoda), snails, sea slugs; cephalopods (Cephalopoda), squids, octopuses; scaphopods (Scaphopoda), tooth shells; Polyplacophora, chitons.

There are a few other mollusc classes.

Blood clotting encompasses a sequence of chemical reactions whose respective products are enzymes that catalyze the following reactions (that is why the clotting reactions are called cascade reactions). In the plasma thromboplastinogen transforms into thromboplastin, a reaction triggered by tissue and platelet factors liberated after injury of the blood vessel. Thromboplastin then catalyzes along  with calcium ions the transformation of prothrombin into thrombin. Thrombin then catalyzes a reaction that produces fibrin from fibrinogen. Fibrin, as an insoluble substance, precipitates to form a network that traps red blood cells and platelets forming the blood clot and containing the hemorrhage.

Synapses are the structures that transmit the neural impulse between two neurons.

When the electric impulse arrives the presynaptic membrane of the axon releases neurotransmitters that bind to postsynaptic receptors of the dendrites of the next cell. The activated state of these receptors alters the permeability of the dendritic membrane and the electric depolarization propagates along the neuron plasma membrane to its axon.

From the mother to the fetus the main transferred substances through the placenta are water, oxygen, nutrients and antibodies. From the fetus to the mother, metabolic wastes, including urea (nitrogen waste), and carbon dioxide.

The main human diseases caused by fungi in immunocompetent patients are coccidioidomycosis, histoplasmosis, blastomycosis, paracoccidioidomycosis, or South American blastomycosis, sporotrichosis and onychomycosis (nail mycosis).

In immuno-deficient patients, besides the diseases mentioned above, other fungal diseases like systemic candidiasis, aspergillosis, cryptococcosis and other opportunistic diseases can occur. 

The body of gastropods is divided into three main portions: head, the visceral mass and foot.

Factor VIII has the function of activating factor X that in its turn is necessary for the transformation of prothrombin into thrombin in the clotting cascade. Hemophilia A is the X-linked genetic disease in which the individual does not produce factor VIII and so is more susceptible to severe hemorrhages.

Most of the neurons are situated within the brain and the spinal cord (central nervous system) in places known as neural nuclei. Neural ganglia, or simply ganglia, are structures of the peripheral nervous system located beside the spinal column or near some organs where neuron cell bodies are also located.

Neurons situated at specific points can present distant axonal terminations and they also can receive impulses from axons of distant neurons. The inferior motor neurons situated in the spinal cord are examples since their axons can transmit information to the extremities of the inferior limbs triggering contractions of the foot.

The umbilical cord is a set of blood vessels that connects the fetus with the placenta. In the fetus one extremity of the cord inserts into the center of the abdominal wall (the later scar of this insertion is the umbilicus or navel). The function of the umbilical cord is to allow the transport of substances, nutrients, gases and residuals, between the fetus and the mother’s body.

The topical or systemic azoles (like itraconazole, fluconazole and others), amphotericin B, the echinocandins (caspofungin, micafungin), terbinafine and griseofulvin are examples of antifungal drugs.

Molluscs present extracellular digestion and a complete digestive system, with mouth and anus.

Clotting factors are substances (enzymes, coenzymes, reagents) necessary for the clotting stages to happen. Besides those triggering factors and reagents already described (tissue and platelet factors, thromplastinogen, prothrombin, fibrinogen, calciumions), other substances participate in the blood clotting process as clotting factors, like factor VIII, whose deficiency causes hemophilia A, or the factor IX, whose deficiency causes hemophilia B.

There are three types of neurons: afferent neurons, efferent neurons and interneurons. Afferent neurons are those that only transmit sensory information from the tissues to neural nuclei and ganglia (where they make connection with interneurons or effector neurons). Efferent neurons are those that transmit commands to tasks performed in several parts of the body. Interneurons, also known as association neurons or relay neurons, serve as connection between two other neurons.

Afference is the conduction of sensory impulses and efference is the conduction of effector impulses  (impulses that command some body action).

Under normal conditions, there is no passage of cells across the placenta during gestation. The placenta has a smooth mucosa separating the richly vascularized region in contact with the mother’s endometrium from the umbilical cord in contact with the fetal blood. This barrier is known as placental barrier. Although permeable to some substances (selective permeability) the placental barrier forbids the passage of cells.

Besides aneuploidies there are other genetic diseases, other chromosomal abnormalities and also the genetic mutations. 

The etiological agent of moniliasis is Candida albicans, a fungus. Moniliasis is also known as mucocutaneous candidiasis. In AIDS moniliasis can complicate and turn into systemic candidiasis, affecting many organs.

Newborns do not yet have their immune system working with complete efficiency and thus they are more susceptible to candidiasis that generally appears in mouth and in the genital mucosae and disappears naturally.

Some molluscs have a tongue-like structure with harsh saliences similar to small teeth. This structure is called radula. The radula projects outside and helps to scrape food.

Hemophilia is medically treated with administration of factor VIII, in case of hemophilia A, or of factor IX, in case of hemophilia B, by means of blood or fresh frozen plasma transfusions. Hemophilia, A or B, is an X linked recessive inheritance and for a girl to be hemophilic it is necessary for both of her X chromosomes to be affected while boys, that have only one X chromosome, are more easily affected. A girl with only one affected chromosome does not present the disease since the normal gene of the unaffected other X chromosome produces the clotting factor.

The peripheral nervous system comprehends the nerves and ganglia of the body.

The placenta has endocrine function since it secretes the hormones progesterone and estrogen that maintain the endometrium (internal covering of the uterus) and prevent menses during pregnancy. The placenta also secretes other important hormones for pregnancy regulation.

Genetic mutations are alterations of the genetic material (compared to the normal condition of the species) involving modifications in the normal nucleotide sequence of a gene but without structural or numeric chromosomal changes.

These modifications may be deletions (loss of nucleotides), substitutions (exchange of nucleotides by other different nucleotides) or insertions (placement of additional nucleotides in the DNA molecule).

The main viral diseases transmitted by respiratory secretions (sneezes, cough) and by saliva drops are flu, mumps, smallpox (variola, already considered eradicated), rubella, measles, SARS. Main viral diseases transmitted through blood or sexual contact are AIDS, hepatitis B, hepatitis C, HPV, ebola hemorrhagic fever. Main viral diseases transmitted by animal vectors are rabies, dengue fever, yellow fever.

Some viral diseases transmitted by fecal-oral route, including contaminated food, are hepatitis A, poliomyelitis (disease almost eradicated in many parts of the world).

Aquatic molluscs oxygenate their blood through gills in direct contact with water. In terrestrial molluscs the rich vascularity under the mantle cavity absorbs air doing the role of a primitive lung.

Most of the clotting factors are produced in the liver. Vitamin K participates in the activation of several clotting factors and it is fundamental for the well-functioning of the blood coagulation.

The function of the myelin sheath is to improve the safety and speed of the neural impulse transmission along the axon. The myelin sheath serves as an electrical insulator preventing the dispersion of the impulse to other adjacent structures. Since the myelin sheath has gaps called Ranviers’ nodes in its length, the neural impulse “jumps” from one node to another thus increasing the speed of the neural transmission.

Not all neurons have a myelin sheath. There are myelinated axonal fibers and unmyelinated ones.

The kingdom Plantae is divided into two big subkingdoms: the bryophytes and the tracheophytes (pteridophytes, gymnosperms and angiosperms). The criterion for the division is the presence or not of conductive (vascular) tissue.

Not every gene mutation causes alteration in the composition of the protein the gene codifies. Since the genetic code is degenerated, i.e., there are amino acids codified by more than one different DNA nucleotide triplet, if by chance the mutation substitutes one or more nucleotides of a codifier triplet and the newly formed triplet still codifies the same amino acid codified by the original triplet there will be no modification in the protein made from the gene. 

If occurring during gestation rubella is a dangerous disease because the virus crosses the placenta and contaminates the fetus. The fetus then develops congenital rubella, a teratogenic (cause of malformations) disease. Congenital rubella may be prevented by vaccination. Doctor must always be consulted before vaccination. 

Molluscs, with the exception of cephalopods, have an open circulatory system, i.e., the blood circulates within vessels, from the heart, but it also fills open lacunas or cavities. In annelids the circulatory system is closed, i.e., the blood circulates only inside blood vessels.

Since hemophilic patients need frequent transfusions of clotting factors (VIII or IX) they are more susceptible to contamination by infectious agents present in the blood from which the transfused elements come. In the past the blood banks did not usually perform HIV detection tests and many hemophilic patients have become infected with the virus.

Axons extend throughout the body inside nerves. Nerves are axon- containing structures presenting many axons and covered by connective tissue. The nerves connect neural nuclei and ganglia with the tissues.

Nerves may contain only sensory axons (sensory nerves), only motor axons (motor neurons) or both types of axons (mixed nerves).

The typical plant cells are eukaryotic (have nucleus), autotrophic (produce their own food) and photosynthetic (use light to make food). Plant cells also have chloroplasts and a cell wall (a structure exterior to the plasma membrane) made of cellulose.

Mutagenic agents, or mutagens, are physical, chemical or biological factors that can cause alteration in DNA molecules.

Examples of well-known or believed to be mutagenic agents are: X, alpha, beta and gamma rays, ultraviolet radiation, nitrous acid, many dyes, some sweeteners, some herbicides, many substances of tobacco, some viruses, like HPV, etc. Small DNA fragments known as transposons can also act as mutagens when incorporated into other DNA molecules.

The vaccines used against poliomyelitis are the Sabin vaccine and the Salk vaccine. The Sabin vaccine contains attenuated virus and is taken through oral drops. The Salk vaccine is made of dead virus and is administered by injection. 

In cephalopods the circulatory system is closed and this provides more speed and pressure for the blood circulation allowing the existence of species with large bodies, like octopuses and giant squids.

Anticoagulants are substances that block the clotting reactions and thus stop the coagulation process. Ordinarily there are anticoagulants circulating in the plasma since under normal conditions the blood must be kept fluid. In Medicine anticoagulants like heparin are used in surgeries in which tissue injuries made by the surgical act could trigger undesirable systemic blood clotting. They are also used to avoid the formation of thrombus inside blood vessels of patients facing increased thrombotic risk.

Ganglia (singular ganglion), or neural ganglia, are structures located outside the central nervous system (for example, beside the spinal column or near viscera) made of concentration of neuron bodies.

Examples of neural ganglia are the ganglia that concentrate cell bodies of sensory neurons in the dorsal roots of the spinal cord and the ganglia of the myenteric plexus responsible for the peristaltic movements of the digestive tube.

In the central nervous system (CNS) the concentrations of neuron bodies are called nuclei and not ganglia.

Bryophytes are nonvascular plants (mosses, liverworts, hornworts), i.e., they do not have a conductive system for transport of sugar, water and nutrients. Tracheophyte plants are vascular plants, they have conductive structures.

Molluscs have one or two pair of spongelike nephridia, similar to kidneys.

Dicoumarol is an anticoagulant drug. Due to its molecular structure dicoumarol competes with vitamin K for the binding to substrates blocking the formation of clotting factors and interrupting the making of prothrombin. Dicoumarol is found in some vegetables undergoing decomposition, and it can cause severe internal hemorrhages when those vegetables are accidentally ingested. Coumarinic anticoagulants cannot be administered during pregnancy since they pass the placental barrier and can cause fetal hemorrhages.

In the central nervous system (CNS) the myelin sheath is made by apposition of oligodendrocyte membranes. Each oligodendrocyte can cover portions of axons of several different neurons. In the peripheral nervous system (PNS) the myelin sheath is made by consecutive Schwann cell membranes covering segments of a single axon. The Ranviers’ nodes appear in the intercellular space between these cells. The myelin sheath is rich in lipids but it also contains proteins.

While plant cells are eukaryotic, autotrophic, photosynthetic and have chloroplasts and cell wall, the animal cells are eukaryotic, heterotroph and do not present chloroplasts nor cell wall.

Examples of recessive genetic diseases are: cystic fibrosis, albinism, phenylketonuria, galactosemia, Tay- Sachs disease. 

Some molluscs, like bivalves and gastropods, secrete an external calcareous carapace (the shell). Calcium carbonate is secreted by the mantle, a fold in the epidermis, forming the shell.

In pelecypods, the shell is made of two valves that articulate and contain the individual body within. In gastropods the shell contains only part of the body, the visceral mass.

Substances known as fibrinolytics, like streptokinase and urokinase, can destroy thrombi (clots formed inside blood vessels, capillaries or within the heart chambers) and are used in the treatment of obstructions of the coronary arteries or other blood vessels. Streptokinase destroys the fibrin network and so it dissolves the thrombotic clot. Its name comes after the bacteria that produce it, the streptococci.

The concept of brain, or encephalon, comprehends the cerebrum (mostly referred to as the hemispheres, but actually the concept also includes the thalamus and the hypothalamus), the brainstem (midbrain, pons and medulla) and the cerebellum. Brain and spinal cord form the central nervous system (CNS).

Bryophytes are nonvascular plants (mosses, liverworts, hornworts), i.e., they do not have a conductive system for transport of sugar, water and nutrients. Tracheophyte plants are vascular plants, they have conductive structures.

The exposition of a population to mutagenic agents (for example, the people living in the surrounds of the Chernobyl nuclear power plant and exposed to the radiation from the nuclear accident in 1986) increases the cancer incidence in that population. This occurs because the mutagenic agents increase the rate of mutation and the probability of mutant cells to proliferate in pathological manner (cancer).

Cancer itself is not a hereditarily transmissible disease. Genetic predispositions for the development of cancer, however, can be inherited.

There are many types of viral hepatitis. The most important epidemiologically are hepatitis A, hepatitis B and hepatitis C.

Octopus and squids generally do not produce external shell (some squid species can have an internal shell). One cephalopod group, the nautiluses, produces an external spiral shell.

Metabolism is the set of physical and chemical processes upon which the life of the cells of a living being depends.

Multiple sclerosis is a severe disease caused by progressive destruction of the myelin sheath in the central nervous system. The Guillain-Barré disease is due to destruction of the myelin sheath in the peripheral nervous system caused by autoimmunity (attack by the own immune system). The genetic deficiency in the formation or preservation of the myelin sheath is an X-linked inheritance called adrenoleukodystrophy. The movie “Lorenzo’s Oil” featured a boy with this disease and his father's dramatic search for treatment.

There are enzymes within the cells that detect errors or alterations in DNA molecules and begin a repair of those errors. First, enzymes known as restriction endonucleases, specialized in cutting DNA molecules (also used in genetic engineering), cut the affected piece of DNA. Then polymerase enzymes build correct sequences of nucleotides correspondent to the affected piece taking as template the DNA chain complementary to the affected chain. Finally the new correct sequence is bound in the DNA under repair by specific enzymes. 

Arthropods present exoskeleton made of chitin. Echinoderms do not have exoskeleton but they present a calcareous endoskeleton. Some chordates also have an external carapace.

Metabolism comprises two opposing processes: anabolism and catabolism. Anabolism is a set of synthesis reactions that transform simpler compounds into organic molecules in general with energy spending. Catabolism is a set of reactions that break organic molecules into simpler and less complex substances in general with liberation of energy. The energy liberated in catabolism may be used in vital processes of the organism, including anabolism.

The cerebrum is divided into two cerebral hemispheres, the right and the left. Each hemisphere is made of four cerebral lobes: frontal lobe, parietal lobe, temporal lobe and occipital lobe. Each cerebral lobe contains the gray matter and the white matter. The gray matter is the outer portion and it is made of neuron bodies; the gray matter is also known as the cerebral cortex. The white matter is the inner portion and it is white because it is in the region where axons of the cortical neurons pass.

Plants have specialized organs (like reproductive organs, roots, limbs, leaves) and differentiated tissues (vascular tissue in tracheophytes, support tissue, parenchyma, etc).

Hepatitis A is an acute disease of low mortality caused by the hepatitis A virus (an RNA virus). It is transmitted by fecal-oral route often through contamination of foods like vegetables and sea-food. The virus attacks the liver and its incubation period varies between 15 and 45 days. After incubation the symptoms are fever, headache, abdominal pain, malaise, nausea and vomiting and the main signs are jaundice, hepatomegaly (enlargement of the liver) and darkened urine (due to excessive bile in blood). Blood tests show increased levels of hepatic enzymes caused by injuries to liver cells. Hepatitis A often heals naturally after 4 to 6 weeks.

The mollusc foot has the function of locomotion, support, fixation, digging in the environment and sometimes of holding prey.

The terminations of the names given to the main mollusc classes come from the Greek word “podos” that means foot.

Gastropods have feet in their ventral region (as the name indicates); pelecypods have ax-shaped feet (ax in Greek is “pelekys”); in cephalopods the feet are near the head.

Homeostasis comprises the processes by which the organism maintains adequate intra and extracellular conditions to keep possible the normal reactions of the metabolism. Homeostatic sensors are structures that detect information from the inner and outer environment of the body. These sensors may be nervous receptor cells, cytoplasmic or membrane proteins and other specialized molecules. Controllers are structures responsible for processing and interpreting information received from the sensors. Controllers in general are specialized regions of the central nervous system but on the molecular level there are also some of them, like DNA, a molecule that can get information from proteins to inhibit or stimulate the expression of some genes. Effectors are elements commanded by the controllers that have the function of bringing about actions that in fact regulate and maintain the equilibrium of the organism, like muscles, glands, cellular organelles, etc., and in the molecular level structures that participate in the genetic translation, the produced proteins, etc.

Meninges are the membranes that enclose and protect the central nervous system (CNS). Cerebrospinal fluid is the fluid that separates the three layers that form the meninges and it has the functions of nutrient transport, defense and mechanical protection for the CNS. The cerebrospinal fluid fills and protects cavities of the brain and the spinal cord.

Cryptogamic (hidden sex organs) plants are those that do not present flowers or  seeds. They comprise the bryophytes and the pteridophytes. Phanerogamic plants are those having seeds. They comprise the gymnosperms and the angiosperms.

Consanguineal marriage is the marriage between relatives, i.e., people having common near ancestors.

The consanguineal marriage increases the probability of recessive genetic disease in the offspring since it is common for people from the same genetic lineage to be heterozygous carriers of alleles that condition recessive genetic diseases.

Molluscs have well-developed sensory structures. It is accepted that cephalopods, like octopus and squid, have eyes with image formation. Snails have antennae and a pair of well- defined eyes. Bivalves do not present eyes but they have photosensitive and tactile cells.

Cephalization is evident in molluscs and neurons concentrate in a ganglial pattern.

The homeostatic maintenance of the body mostly occurs by means of alternating antagonistic compensatory mechanisms. There are regulators that lower the pH and others that increase it, there are effectors whose function is to increase the body temperature and others that lower it, hormones exist that, e.g., reduce the level of glucose in the blood and others that increase the glycemic level. The use of antagonistic mechanisms is a strategy found by evolution to solve the problem of the maintenance of the body equilibrium. 

In the central nervous system the cerebellum is the main controller of the motor coordination and equilibrium of the body. (Do not confuse this with muscle command, performed by the cerebral hemispheres).

The angiosperms are divided into monocotyledonous and dicotyledonous. (These categories are explained later in this text.)

Allele frequency is the percentage of appearances of an allele in the genotypes of a given population (compared to the other alleles of the studied gene). For example, in the ABO blood system there are three alleles (I_A, I_B and i). Considering a group of three persons, one with genotype I_Ai, other andI_AI_B other ii, and other ii, the frequency of the allele I_A in this “population” is 2/6, the frequency of the allele I_B is 1/6 and the frequency of the allele i is 3/6.

The main human viral infections transmitted by mosquitoes are dengue fever and yellow fever.

Molluscs are important players in several food chains in ecosystems. Many marine molluscs are part of a common human diet, like octopus and squid, very appreciated in the East, and oysters and mussels, consumed all around the world. Besides molluscs that are part of the food industry, pearls made by oysters have high commercial value.

Negative feedback happens when the response to a given action generates an effect that inhibits that action. For example, when the carbon dioxide concentration in blood is high the pulmonary respiration is stimulated for the CO₂ excess to be expelled through expiration. Hyperventilation, however, lowers the carbon dioxide concentration in blood too much generating a negative feedback that commands the reduction of the respiratory frequency. Negative feedback is the main mechanism of homeostasis and it occurs in a variety of processes, such as in blood pressure control, glycemic control, muscle contraction, etc.

The cerebellum is the main brain structure that coordinates the movement and the equilibrium of the body. For this reason it appears more developed in mammals that jump or fly (like bats). The cerebellum is also very important for the flight of birds.

Genetic equilibrium is the result of the Hardy-Weinberg law, a principle that affirms that under specific conditions the frequencies of the alleles of a gene in a given population remain constant.

(The Hardy-Weinberg principle is not valid in the following conditions: for populations too small, in the occurrence of noncasual (driven) crossings, for populations with many infertile members and in case of action of evolutionary factors, like natural selection, mutations and migrations.)

Hepatitis C is caused by an RNA virus and transmitted through blood (like hepatitis B, through wounds, sexual relations, transfusions, accidents with contaminated material, etc.). The usual manifestation is chronic and many people that have the virus are asymptomatic. The liver gradually undergoes injuries and some patients evolve to cirrhosis, hepatic cancer and death. Today interferon use is an attempt to control replication of the hepatitis C virus. The assessment of the disease is made periodically by hepatic biopsy and by blood tests of hepatic enzymes. Viral replication can be estimated through PCR (polymerase chain reaction).

Examples of representing beings: snails, sea slugs, octopuses, squids, mussels, oysters. Basic morphology: soft body, with or without calcareous shell. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, coelomates.

Digestive system: complete. Respiratory system: branchial or, in terrestrial gastropods, lungs. Circulatory system: open or, in cephalopods, closed.

Excretory system: nephridia. Nervous system: ganglial, advanced cephalization in cephalopods. Types of reproduction: sexual.

In positive feedback the effect caused by an action stimulates the action even more. This is a rarer mechanism of the homeostatic regulation. An example of positive feedback is the blood clotting process in which each chemical reaction produces enzymes that catalyze the following reaction until the formation of fibrin. Therefore the products of the antecedent chemical reactions are consumed and the equilibrium of each reaction is dislocated towards the production of more enzymes (a positive feedback).

The neural regulation of breathing, blood pressure and other physiological parameters like heartbeat, digestive secretions, peristaltic movements and transpiration is performed by the medulla. The medulla, together with the pons and the midbrain, is part of the brainstem.

Sexual reproduction may take place through three different types of life cycles: the haplontic (the being is haploid) haplobiontic (a single type of being) cycle; the diplontic (the being is haploid) haplobiontic (a single type of being) cycle; and the diplobiontic cycle (two types of beings, one haploid and the other diploid). The diblobiontic cycle is known as alternation of generations, or metagenesis. In humans the cycle is diplontic haplobiotic (a single diploid being).

Genetic disease may be diagnosed in the prenatal period by karyotype analysis, in case of aneuploidies, or by DNA analysis, in case of other diseases.

The test is performed by removal of material containing cells of the embryo by amniocentesis (extraction of amniotic fluid) or cordocentesis (puncture of the umbilical cord) or even by chorionic villus biopsy (that can be done earlier in gestation).

Ultrasonography is a diagnostic procedure for some genetic diseases that produce morphological variations during the embryonic development. The study of genetic family trees is also an important auxiliary method in the early diagnosis of many genetic diseases.

Dengue, or dengue fever, is an epidemic disease in some countries (for example, in Brazil), and its most dangerous form is hemorrhagic dengue. It is caused by four different but related viruses, the reason that it is difficult for an already infected person to become immunized against the disease. Dengue is prevalent in tropical and subtropical regions of Asia, Africa and South America and it is transmitted by the bite of the Aedes aegypti mosquito, a diurnal mosquito.

The viral infection causes varied symptoms similar to flu, like fever, malaise, myalgia (muscle pain); in hemorrhagic dengue high fever, with or without convulsions, and hemorrhages may occur leading to circulatory failure and death. There is no vaccine against dengue since there are four agent viruses and it is supposed that the severity of the disease is greater when the patient is already protected against one type of virus and is infected by another. The best prophylactic method is to eliminate the vector mosquito.

Pearls are made from small strange particles that deposit between the shell and the mantle of the oyster. These particles trigger a defense process by the organism and they are gradually covered by calcium carbonate layers secreted by the oyster thus giving birth to pearls.

In the artificial production of pearls a small fragment of shell covered with mantle pieces is inserted between the shell and the mantle of an oyster and a pearl is formed around the graft.

A nutrient is every substance used in the metabolism and which is acquired from the diet. For example, vitamins and essential amino acids are nutrients.

In the brain conscious sensory information is received by the neurons situated in a special region called postcentral gyrus (or sensory gyrus). Gyri are the convolutions of the cerebrum. Each of the two postcentral gyri are located in one of the parietal lobes of the cerebrum. The voluntary motor activity (voluntary muscle movement) is commanded by neurons situated in the precentral gyrus (or motor gyrus). Each of the two precentral gyri are located in one of the frontal lobes of the cerebrum. The names post- and pre-central refer to the fact that the motor and sensory gyri are spaced apart in each cerebral hemisphere by the sulcus centralis, a fissure that separates the parietal and frontal lobes.

In the plant life cycle (diplobiontic life cycle) and in the haplontic haplobiontic life cycle gametes are made by mitosis and not by meiosis. Obviously in some stage of these sexual life cycles meiosis must occur.

The phenomenon in which a large number of genes is destroyed or introduced in a population is called genetic drift. When a genetic drift occurs the Hardy Weinberg principle is not applicable.

It is difficult to make vaccines against dengue because there are four different types of viruses that cause the disease (DEN1, DEN2, DEN3 and DEN4) and it is supposed that the protection against one of them aggravates the clinical manifestation when the person is later infected by the another dengue virus.

In the case of HIV, the production of a vaccine is difficult because the virus is highly mutant and evades the antibody action. 

The echinoderm skeleton is internal, i.e., it is an endoskeleton. It is made of calcium carbonate (calcareous).

Vertebrates also have internal skeleton made of bones and cartilages.

Arthropods have an external carapace made of chitin, a chitinous exoskeleton. Some molluscs present a calcareous shell that works as exoskeleton.

The classification criterion of nutrients into macro and micronutrients has no relation to the size of the molecule. Macronutrients are those needed in great amount, for example, proteins and carbohydrates. Micronutrients are those needed in small quantities, like vitamins.

The spinal cord is the dorsal neural cord of vertebrates. It is the part of the central nervous system that continues in the trunk to facilitate the nervous integration of the whole body.

The spinal cord is made of groups of neurons situated in its central portion forming the gray matter and of axon fibers in its exterior portion forming the white matter. Neural bundles connect to both lateral sides of the spinal cord segments to form the dorsal and ventral spinal roots that join to form the spinal nerves. The dorsal spinal roots present a ganglion with neurons that receive sensory information; the ventral spinal roots contain motor fibers. Therefore the dorsal roots are sensory roots and the ventral roots are motor roots.

There are asexual forms of reproduction in plants. Some naturally detached pieces of root, limbs or leaves develop into another complete individual. Artificial asexual reproduction of plants can be obtained by means of grafting or cutting.

Considering p the frequency of one of the alleles and q the frequency of the other allele of a given gene in a population, in this population individuals produce p gametes with the first allele for each q gamete containing the second allele. Therefore the probabilities of formation of homozygous genotype for the first allele is p², of homozygous genotype for the second allele is q² and of the heterozygous genotype is p.q + q.p, i.e., 2p.q.

Since the sum of those probabilities necessarily is 1, the resulting mathematical expression is: p² + 2p.q + q² = 1.

In general the number of genotypical forms is not identical to the number of phenotypical forms since there are dominance and other interactions between genes that affect the manifestation of the phenotype.

Yellow fever is a viral infection that occurs mainly in Central Africa and in the Amazon region of South America. It is prevented through vaccination and transmitted by many species of mosquitoes of the Aedes genus, including Aedes aegypti e Haemogogus. The infection causes clinical manifestations that range from asymptomatic cases to lethal fulminating cases. Generally the disease begins with fever, chills, malaise, headache, nausea and evolves to jaundice (increase of bilirubin in blood, after which the disease is named), mucosal and internal hemorrhages, hemorrhagic vomiting and renal failure. 

Prevention is done by regular mass vaccination and vaccination of travelers to endemic areas. The combat against the vector mosquito is also an important prophylactic measure.

Echinoderms are marine animals, they live in salt water.

One possible and utile functional classification for nutrients is the one that separates them into energetic, structural and regulatory. Energetic nutrients are those used as energy source for the metabolism; mainly they are the carbohydrates (but fats and proteins can also be converted into acetyl-CoA and “cycle” the Krebs  cycle). Structural nutrients are those used in the support and structure of cells and tissues; they are the amino acids that form structural proteins, like collagen, the membrane proteins, the cytoskeleton proteins, the contractile proteins of the muscle tissue, etc. Regulatory nutrients are those that constitute enzymes and coenzymes of the homeostasis, metabolites of the osmotic and electrolytic equilibrium of cells and hormones; some amino acids, vitamins and mineral salts are part of this group.

According to researchers some of the main regions of the nervous system associated with the memory phenomenon are the hippocampus, situated in the interior portion of the temporal lobes, and the frontal lobe cortex, both part of the cerebral hemispheres.

The plant life cycle is known as alternation of generations because in this cycle there are two different forms of living beings that alternate each other, one haploid and the other diploid. Alternation of generations is also called the diplobiontic cycle, or metagenesis, and it does not occur only in plants, other living beings, like cnidarians, present the cycle.

Individuals that carry a same genotype do not always manifest in an identical manner the correspondent phenotype. These manifestations may differ in intensity, from one individual to another, or even the phenotype may not manifest in some percentage of carriers.

Gene penetrance is the percentage of phenotypical manifestation of a gene in a given population of carrier individuals (same genotype). Gene expressivity is the degree (intensity) of the phenotypical manifestation of a gene in each individual or group of individuals that carry the gene (same genotype). The gene penetrance and the gene expressiveness may be influenced by the environment.

Antibacterial drugs, potent against a great variety of bacteria, are not effective against viruses, which are intracellular parasites. A viral infection is difficult to treat since anti-viral drugs are too specific and have limited efficiency. In general the anti-viral drugs reduce the viral load (number of virus) relieving symptoms. The anti- virals (and anti-retrovirals, drugs that act against RNA virals) often inhibit the action of specific enzymes that participate in the virus life cycle. 

Echinoderms, as the name indicates (echino = spiny, derma = skin), are creatures with spines originated from an endoskeleton. Their endoskeleton is made of calcareous plaques that besides spines contain pedicellaria, small pincers used to clean the body and to help the capturing of prey. They also present a hydrovascular system known as the ambulacral system. Adult echinoderms have pentaradial symmetry; the radial symmetry in these animals is secondary, present only in adults.

Most vitamins are coenzymes (fundamental substances for the enzyme functioning) that are not produced by the organism and must be obtained from the diet. The main vitamins needed by humans are vitamins A, C, D, E, K, the vitamins of the B complex (including folic acid), biotin and pantothenic acid.

In the haplontic haplobiontic life cycle the single and lasting form is haploid. In the diplontic haplobiontic life cycle it is diploid. In the diplobiontic life cycle the lasting individual that alternates with the intermediate form may be the haploid gametophyte (as in bryophytes) or the diploid sporophyte (as in pteridophytes).

For a trait conditioned by two alleles, for example, A and a, a balanced frequency between the alleles A and b is more advantageous for survival. For example, in an environmental situation in which the aa homozygous phenotype becomes incompatible with life the presence of a good number of individuals AA and Aa will result in better survival chance for the species. Another example: an environmental situation in which the dominant phenotype becomes incompatible with life; in this case the existence of heterozygous and recessive homozygous individuals in enough number may be fundamental for the survival of the species.

HIV (human immunodeficiency virus) is supposed to be transmitted through blood, semen, vaginal secretions and maternal milk.

HIV is the virus that causes AIDS (acquired immune deficiency syndrome), a disease characterized by destruction of cells of the immune system making the body susceptible to many opportunistic and severe diseases.

The system that permits movement and fixation to substrates in echinoderms is called the ambulacral system. In these animals water enters through a structure called madreporite, passes through channels and reaches the ambulacral feet in the undersurface of the body. In the ambulacral region in contact with the substrate, there are tube feet filled and emptied by water thus acting as suckers.

Water-soluble vitamins are those soluble in water. Fat-soluble vitamins are those soluble in oil (lipids, fat). Vitamin C and the vitamins of the B complex are examples of water-soluble vitamins. Vitamins A, D, E and K are examples of fat-soluble vitamins. Fat-soluble vitamins, since they are not soluble in water, cannot easily be excreted by the body. So they tend to accumulate in tissues with toxic effect when they are ingested in amounts over what is necessary.

Bryophytes are nonvascular plants, i.e., they do not have conductive tissues and they perform transport of water and nutrients by diffusion; they are cryptogamic, i.e., they do not present flowers or seeds; they are small in size; they present water-dependant fecundation; in their life cycle the lasting form is haploid (the gametophyte) and the sporophyte depends on the gametophyte to survive.

Biotechnology is the application of biological knowledge to obtain new techniques, materials and compounds of pharmaceutical, medical, agrarian, industrial and scientific use, i.e., of practical use.

The pioneer fields of biotechnology were agriculture and the food industry but nowadays many other practical fields use its techniques.

CD4 lymphocytes are T helper lymphocytes that present in their plasma membrane receptor proteins called CD4. CD4 lymphocytes are the cells that HIV infects and within which the virus replicates. HIV has proteins in its capsule that bind to the CD4 receptors of lymphocytes. Through that bond the virus fuses with the cell membrane and its content (RNA, reverse transcriptase, protease, etc.) penetrates into the cytoplasm and the viral replication process begins.

HIV RNA is then converted into DNA by the reverse transcriptase. The new DNA is inserted into the genetic material of the lymphocyte with the aid of enzymes called integrases. By transcription and translation this DNA commands the synthesis of proteins necessary for the assemblage of new viruses. Long polypeptides are thus produced and then fragmented into proteins and viral enzymes by the enzyme protease. So new HIV viruses are assembled and break the cell membrane to gain the circulation.

Echinoderms present a complete digestive system, with mouth and anus.

Deficiency of vitamin A (retinol) may cause night blindness, corneal dryness (xerophthalmia) and predisposition to skin injuries.

In the physiology of vision, vitamin A participates in the formation of rhodopsin, a pigment responsible for the visual perception in less illuminated places.

Nutrition Vitamins - Image Diversity: vitamin A deficiency

The sensory fiber that first conducts the arch reflex connects with neurons of the arch reflex but it also connects with secondary sensory neurons of the spinal cord that transmit information upwards to other neurons of the brain. This is obvious since the person that received the initial stimulus (e.g., the percussion on his/her kneecap) perceives it (meaning that the brain became conscious of the fact).

As in all plants the life cycle of bryophytes is diplobiontic (alternation of generations). In bryophytes the lasting form is the haploid one.

Genetic engineering is the use of genetic knowledge to artificially manipulate genes: It is one of the fields of biotechnology. 

HIV is a retrovirus, i.e., an RNA viral (its genetic material is RNA and not DNA). Reverse transcriptase is a specific enzyme of the retrovirus responsible for the transcription of the viral RNA into DNA within the infected (host) cell. This DNA then commands the production of viral proteins and the viral replication.

Sea urchins have a teeth-like structure juxtaposed to the mouth and made of five teeth connected to ossicles and muscle fibers. This structure, known as Aristotle’s lantern, is use to scratch food, mainly algae, from marine rocks.

The folic acid (when ionized it is called folate) is a coenzyme that participates in the synthesis and duplication of DNA and for this reason it is fundamental for cell division. If there are not enough folic acid in cells with great turnover, like red blood cells, they have their production reduced. In folic acid deficiency precursor cells (reticulocytes) that would originate erythrocytes (red blood cells) begin cell division but the process is very slow while the cytoplasm growth is normal. So the cells became abnormally large, a typical feature of this kind of anemia called megaloblastic anemia. Megaloblastic anemia can be caused also by vitamin B12 (cyanocobalamin) deficiency since this vitamin is important for cell division too. Both types of anemia are nutrient deficiency anemias. Nutrition Vitamins - Image Diversity: megaloblastic red blood cell

In the arch reflex first a sensory neuron located in the ganglion of a dorsal spinal root collects the stimulus information from the tissues. This sensory neuron makes direct or indirect (through interneurons) connection with inferior motor neurons of the spinal cord. These motor neurons then command the reflex reaction. So sensory neurons, interneurons and inferior motor neurons participate in the arch reflex.

The main bryophyte groups are the mosses, the liverworts and the hornworts.

Restriction enzymes, or restriction endonucleases, are enzymes specialized in the cutting of DNA fragments each acting upon specific sites of the DNA molecule. Restriction enzymes are used in the recombinant DNA technology to obtain with precision pieces of DNA molecules to be later inserted into other DNA molecules cut by the same enzymes.

It is difficult to produce a vaccine against AIDS because the HIV is a highly mutant virus. In almost every replication the produced viruses have different proteins in their surface making difficult their recognition by the immune system.

Echinoderms do not have an excretory system. Their excretions are eliminated by diffusion.

Vitamins of the B complex are: thiamin, or vitamin B1; riboflavin, or vitamin B2, and niacin (B3), essential for the constitution of the hydrogen acceptors FAD, NAD and NADP of the energetic metabolism; pyridoxine, or B6; and cyanocobalamin, or vitamin B12. Deficiency of vitamin B1 causes beriberi, loss of appetite and fatigue. The lack of vitamin B2 causes mucosal injuries in the mouth, tongue and lips. Deficiency of niacin causes nervousness, digestive disturbances, loss of energy and pellagra. Lack of vitamin B6 causes skin lesions, irritation and convulsions. Vitamin B12 acts together with folic acid and its deficiency causes cell division disruptions leading to pernicious anemia (a type of megaloblastic and nutrient deficiency anemia).The absorption of vitamin B12 depends on another substance called the intrinsic factor secreted by the gastric mucosa.

The arch reflex depends only on the integrity of the fibers at a single spinal level. In the arch reflex the motor response to the stimulus is automatic and involuntary and does not depend upon the passage of information to the brain. So it happens even if the spinal cord is damaged at other levels.

The bryophyte sporophyte in general is a tiny long stem that grows on the top of the gametophyte. The sporophyte depends totally upon the gametophyte to obtain nutrients.

The cause of the immunodeficiency presented by AIDS patients is the destruction of CD4 T helper lymphocytes by the HIV. With this destruction the immune system becomes impaired and the body cannot defend itself against many diseases that normally do not develop in immunocompetent people.

Echinoderms and chordates are deuterostomes, i.e., in their embryonic development the blastopore turns into the anus. All other animals with complete digestive system are protostomes, i.e., their blastopore originates the mouth.

The blastopore is the first opening of the digestive tube that appears in the embryonic development.

Vitamin C, or ascorbic acid, participates in the metabolism of collagen and it is fundamental for the integrity of blood capillaries. Scurvy is the disease caused by a lack of vitamin C. It is characterized by tissue lesions in the skin, lips, nose and joints. Scurvy, or scorbutus, was also known as sailors' disease because in maritime voyages of the past it was not common to get on board food that contained vitamin C, like citric fruits. So the sailors became ill with scurvy. 

The gray matter, or gray substance, of the spinal cord contains predominantly neuron bodies (inferior motor neurons, secondary sensory neurons and interneurons). The white matter is mainly made of axons that connect neurons of the brain with spinal neurons.

Like adult amphibians, the bryophytes live in the terrestrial environment but they depend on water to reproduce. For this reason the nickname is justified.

DNA ligases are enzymes specialized in tying the complementary DNA chains that form the DNA double helix. These enzymes are used in the recombinant DNA technology to insert pieces of DNA cut by restriction enzymes into other DNA molecules submitted to the action of the same endonucleases. 

Among the opportunistic diseases that affect HIV infected people during the AIDS stage some are: mucocutaneous and systemic candidiasis, Kaposi’s sarcoma (blood vessel tumors that result in darkened spots on the skin and internal organs), tuberculosis, cytomegalovirus infection, pneumonia by the fungus P. Carinii (pneumocystis pneumonia), toxoplamosis, herpes, etc. Some other opportunistic diseases are: salmonellosis, histoplasmosis, aspergillosis, cryptococcosis, isosporiasis, lymphomas. 

Adult echinoderms, along with cnidarians, are the animals that present radial symmetry, i.e., their body structures are distributed around a center. The radial symmetry in echinoderms however is a secondary radial symmetry, since their larval stage has bilateral symmetry and the radial pattern appears only in adult individuals (there are some few adult echinoderms with lateral symmetry). All other animals have lateral symmetry with exception of poriferans (no symmetry is defined for them).

Echinoderms do not present cephalization and they have a diffuse network of nerves and neurons made of a neural ring around the mouth and of radial nerves that ramify to follow the pentaradial structure of the body.

To obtain vitamin C, for example, from an orange dessert, the vitamin containing food cannot be submitted to high temperatures (cooking) since vitamin C is thermolabile, i.e., it is inactivated by heat.

The sensory fiber that first conducts the arch reflex connects with neurons of the arch reflex but it also connects with secondary sensory neurons of the spinal cord that transmit information upwards to other neurons of the brain. This is obvious since the person that received the initial stimulus (e.g., the percussion on his/her kneecap) perceives it (meaning that the brain became conscious of the fact).

In bryophytes there are no water- conducting or nutrient-conducting structures and the transport of these substances is done by cell to cell diffusion. The small size of bryophytes relates to this feature since if there are no conductive vessels it is not viable to have cells too far from each other (the emergence of the conductive tissues in tracheophytes allowed their increase in size).

Plasmids are circular DNA molecules present in the genetic material of some bacteria. They may contain genes responsible for bacterial resistance to some antibiotics and for proteins that cause virulence (pathogenic hostility). 

A person can be a carrier of the HIV without necessarily being affected by the immunodeficiency syndrome at that time since the virus can remain in the body for many years without producing clinical manifestations. The AIDS condition is characterized when the patient actually becomes immunodeficient and begins to show signs and symptoms of opportunistic diseases. 

In echinoderms there are not well- defined respiratory (with the exception of the holothurian group) and circulatory systems. The ambulacral hydrovascular system plays the function of these systems.

Vitamin D, or calciferol, is synthesized in the skin by the action of the ultraviolet range of sunrays upon precursor molecules. Later it is transformed into its active form in the liver and the kidneys.

The arch reflex depends only on the integrity of the fibers at a single spinal level. In the arch reflex the motor response to the stimulus is automatic and involuntary and does not depend upon the passage of information to the brain. So it happens even if the spinal cord is damaged at other levels.

The better known pteridophytes are the ferns and the maidenhairs, from the filicinae (filicopsida) group, and the selaginellas, mosslike plants from the lycopodineae group (lycopsida). Pteridophytes are cryptogamic plants, i.e., they are flowerless and seedless.

The CD4 counting test is done from a blood sample of a person infected by the HIV. In this test the number of CD4 cells is counted. If that number (concentration) is abnormally low it means immunodeficiency and the patient usually must be treated with antiretroviral drugs. The other test used to follow up the infection is the viral load test. In this test the number of HIV virus is estimated from a blood sample indicating whether the treatment (control) of the disease has been successful or not. 

The fecundation in echinoderms is external, gametes are liberated in water where fecundation occurs.

The majority of echinoderms are dioecious with male and female individuals.

The lack of vitamin D causes the disease known as rickets (rachitis), characterized by decalcification of bones and bone deformities. Vitamin D is fundamental for absorption of calcium and thus it is related to the osseous tissue health.

The poliovirus parasites and destroys spinal motor neurons causing paralysis of the muscles that depend on these neurons.

The incubation period of the HIV (the time interval between the infection and the beginning of the immunodeficiency symptoms) ranges in average from 10 to 15 years.

Acute AIDS is the clinical situation characterized by signs and symptoms that may appear in 5 to 30 days after the infection by the HIV, due to the high replication rate of the virus on this period. Acute AIDS does not always happen and many times it is mistaken for common diseases like the common cold. The patient can present fever, malaise, myalgia (muscle pain) and arthralgia (joint pain), lymphadenomegaly (enlargement of lymphnodes), sweating and diarrhea. The manifestation often lasts from 3 to 15 days.

In echinoderms embryonic development is indirect, with ciliated larvae.

Vitamin E, or tocopherol, is a fat-soluble vitamin that participates as coenzyme in the respiratory chain, the final stage of the aerobic cellular respiration. Its deficiency may cause sterility, spontaneous abortions and muscle dystrophy. Vitamin E is mainly found in corn oil and peanut oil, wheat germ, milk, eggs and leafy vegetables.

The efferences (reactions) of the nervous system can be classified into voluntary, when controlled by the will, and involuntary, those not consciously controlled. Examples of reactions triggered by volition are the movements of the limb, tongue and respiratory muscles. Examples of involuntary efferences are those that command the peristaltic movements, the heartbeat and the arterial wall muscles. The skeletal striated muscles are voluntarily contracted; the cardiac striated and the smooth muscles are involuntarily contracted.

Pteridophytes are tracheophyte (vascular) plants, i.e., they have tissues specialized in conduction of water and nutrients. Bryophytes are nonvascular plants. In pteridophytes therefore the substance transport is done through vessels and in bryophytes that transport occurs by diffusion.

Cloning is the making of an organism genetically identical to another by means of genetic engineering. The basis of cloning is the nucleus transplantation technology. A nucleus from a cell is extracted, generally from an embryonic (not differentiated) cell and this nucleus is inserted into a previously enucleated reproductive cell (in general an egg cell); the egg is then implanted in the organ where the embryonic development will take place. If embryonic development occurs the new organism will have identical genetic patrimony to the organism owner of the cell whose nucleus was used in the transplantation.

The HIV infection is often divided into three phases: the acute phase, from the infection to 1 until 9 weeks after the infection, a phase in which the virus replicates and the number of CD4 lymphocytes is reduced; the asymptomatic phase, lasting from 9 weeks to often more than 10 years, in which the viral load remains stable and the CD4 count is not abnormally low; the AIDS stage, when the viral load is high, CD4 count is abnormally low and opportunistic diseases manifest. 

Examples of representing beings: sea cucumber, sea urchin, starfishes. Basic morphology: calcareous endoskeleton with spines, ambulacral system. Type of symmetry: secondary radial. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete, deuterostomes. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: simple, nerve network without ganglia or cephalization. Types of reproduction: sexual, with larval stage.

The main ions that act in the regulation of the osmotic pressure in cells and tissues are the chlorine anion, the sodium cation and the potassium cation.

Functionally the nervous system can be divided into the somatic nervous system and visceral nervous system.

The somatic nervous system includes the central and peripheral structures that make voluntary control of efferences. Central and peripheral structures that participate in the control of the vegetative (unconscious) functions of the body are included in the concept of visceral nervous system.

The efferent portion of the visceral nervous system is called the autonomic nervous system.

As the first tracheophytes, pteridophytes were also the first plants to extensively colonize the terrestrial environment forming forests. They also constituted an important source of food for terrestrial animals. By presenting conductive vessels they could be larger, a feature inherited from them by phanerogamic plants.

After the infection by the HIV the immune system begins the production of antibodies (primary immune  response) against the virus. The tests ELISA (enzyme-linked immunosorbent assay) and Western blot search for the presence of specific antibodies against HIV antigens in blood samples. Since only one positive ELISA is not conclusive, as false positive tests may happen, the Western-blot test is often used after the positive ELISA.

The five echinoderm classes are: asteroids (starfishes), ophiuroids, crinoids, holothuroids (sea cucumbers) and echinoids (sea urchins and sand dollars).

Deficiency of vitamin K predisposes to hemorrhages since this vitamin is fundamental for the formation of prothrombin in the blood clotting process.

In these plants meiosis takes place within structures called sorus (plural, sori), small dustlike brown dots lining the underside of fern leaves. The sori contain sporangia where reproductive cells undergo meiosis and where spores are produced.

The recombinant DNA technology and the nucleus transplantation technology (cloning) are extremely dangerous since they are able to modify, in a very short time, the ecological balance that evolution has taken millions of years to create on the planet. During the evolutionary process, under the slow and gradual action of mutations, genetic recombinations and of natural selection species emerged and were modified and genetic patrimonies were formed. With genetic engineering however humans can mix and modify genes, making changes of unpredictable long term consequences, risking creating new plant or animal diseases, new types of cancers and new disease outbreaks. It is a field as potentially dangerous as the manipulation of nuclear energy.

The phylum Chordata is divided into protochordates (urochordates and cephalochordates) and vertebrates (cyclostomes, fishes, amphibians, reptiles, birds and mammals).

Besides being important for the osmotic regulation and for the acid-base equilibrium (pH) potassium is fundamental for the excitatory mechanisms of nerves and in muscle contraction.

In general the actions of the sympathetic and the parasympathetic are antagonistic, i.e., while one stimulates something the other inhibits and vice versa. The organs, with few exceptions, get efferences from these two systems and the antagonism between them serves to modulate their effects. For example, the parasympathetic stimulates salivation while the sympathetic inhibits it; the parasympathetic constricts pupils while the sympathetic dilates it; the parasympathetic contracts the bronchi while the sympathetic relaxes them; the parasympathetic excites the genital organs while the parasympathetic inhibits the excitation.

Ferns are constituted by small roots that come downwards from the rhizome (stem, often horizontalized). The fronds also arise from the rhizome. On the back side of each leaf of the plant there are small dustlike dots called sori (singular, “sorus”, also known as “seeds”).

DNA fingerprint, the method of individual identification using DNA, is based on the fact that the DNA of every individual (with exception of identical twins and individual clones) contains nucleotide sequences exclusive to each individual.

Although normal individuals of the same species have the same genes in their chromosomes, each individual has different alleles and even in the inactive portions of the chromosomes (heterochromatin) there are differences in nucleotide sequences among individuals

The main prophylactic measures against HIV infection are: the use of condoms in sexual relations, not to share syringes and needles, careful handling of contaminated medical and nosocomial material, up-to-date information about the virus and its mode of transmission. 

In vertebrates the notochord disappears and gives birth to the spinal column (vertebral column). In protochordates the notochord remains for the rest of the life.

Biotin (also know as vitamin B8) is a vitamin that acts in the metabolism of amino acids and other acids. Pantothenic acid (also known as vitamin B5) is important for the aerobic cellular respiration since it acts in the transport of acetyl and acyl radicals. Biotin is made by bacteria that live in the human digestive tube (under interspecific harmonious ecological interaction) and this supply in general is enough for the body. Biotin and pantothenic acid are found in vegetables, cereals, eggs, fish, milk and lean meat.

Considering the example of invertebrates it is observed that evolution makes the increasing of the complexity of the organisms to be accompanied by convergence of nervous cells to special structures for controlling and commanding: the ganglia and the brain. In simple invertebrates, like cnidarians, the nervous cells are not concentrated but they are found dispersed in the body. In platyhelminthes a beginning of cephalization with the anterior ganglion concentrating neurons is already verified. In annelids and arthropods the existence of a cerebral ganglion is evident. In cephalopod molluscs the cephalization is even greater and the brain commands the nervous system. 

Prothallus is the pteridophyte gametophyte (the haploid individual that forms gametes). The gametophyte develops by mitosis from a spore.

Besides biological perils, a very serious moral problem involves the nucleus transplantation technology concerning humans: an individual right of a human being is offended when a man or woman is made as a copy of another.

Since it is impossible to first ask if the person to be generated wants or not to be a genetic copy of another person, certainly the most important human right is being offended, one's individual freedom, when a human being is obliged to be a genetic copy of another. It is indeed a danger to democracy, whose most basic principle must be non-violation of individual freedom.

Anti-retroviral drugs used in AIDS treatment try to approach any of the several steps of the HIV life cycle. There are three main groups of drugs: the reverse transcriptase inhibitors, that attempt to disallow the formation of DNA from the viral RNA (zidovudine, or AZT, ddI, d4T, 3TC, nevirapine, efavirenz, etc.); the protease inhibitors, that block the enzyme protease necessary for the assemblage of new virus (saquinavir, ritonavir, nelfinavir, indinavir); and the group of entry inhibitors (includes fusion inhibitors), that try to block the entry of HIV into the host cell (enfuvirtide, a fusion inhibitor). 

All beings of the phylum Chordata have branchial clefts in the pharynx (in some species present only in the embryo), notochord (substituted by the spinal column in vertebrates) and dorsal neural tube.

Magnesium, zinc and copper are examples of biological coenzymes.

In vertebrates the nervous system is well-characterized, having the brain and dorsal neural cord protected by rigid skeletal structures. In most invertebrates the nervous system is predominantly ganglial, with ventral neural cords.

Like all plants pteridophytes present diplobiontic (alternation of generations, or metagenesis) life cycle.

It is believed that the earth is approximately 4.5 billion years old.

The treatment of acquired immune deficiency syndrome is often done with one or more anti-retroviral drugs of different groups, for example, a protease inhibitor plus a reverse transcriptase inhibitor in an attempt to approach in one single time two or more stages of the HIV life cycle. Besides that, the high mutation rate of the virus may make the treatment ineffective and for this reason the use of the drug cocktail allows physicians to choose other different drug combinations to escape the viral resistance.

The tubular and dorsal nervous system of chordates is related to the cephalization in these animals and to bilateral symmetry.

The presence of neural integrating centers with concentration of neurons in the brains and the spinal cord (central nervous system, CNS) allowed the increase of the interaction complexity between these animals and the environment. Receptor (afferent conduction) and efferent (motor, regulatory and behavioral reactions) functions come out more sophisticated in chordates due to the presence of more well-developed neural networks.

These features have been preserved by evolution as they provide adaptive advantage to their owners.

Iodine deficiency causes hypothyroidism, an abnormally lower production of thyroid hormones that need iodine to be synthesized.

In vertebrates the brain and the spinal cord are protected by membranes, the meninges, and by osseous structures, respectively the skull and the vertebral column. These protections are fundamental for the integrity of those important organs that command the functioning of the body.

In pteridophytes gametes are made by mitosis from special cells of the gametophyte. As in all plants, in pteridophytes, meiosis is sporic, i.e., cells of the sporophyte undergo meiosis and generate spores that then by mitosis develop into the gametophyte.

From analysis of data collected by the Hubble telescope the age of the universe is estimated to be about 12 billion years. 

Worm infections are human diseases caused by platyhelminthes (flatworms) or nematodes (roundworms).

In humans the branchial clefts located in the anterior region of the pharynx (also known as pharyngeal clefts) are present only in the embryonic stage and disappear later.

Iron acts as a constituent of the hemoglobin molecule and of enzymes of the digestion and energetic metabolism. Dietary iron deficiency causes iron deficiency anemia, abnormal lowering of hemoglobin concentration in blood due to lack of iron. (In pregnancy there is a high consumption of iron by the fetus and this fact can lead to anemia.)

Neurons receive and transmit chemical stimuli through neurotransmitters released in the synapses. Along the neuron body however the impulse transmission is electrical. So neurons conduct electric and chemical stimuli.

Pteridophytes are more common in humid places because they depend on water for their gametes to fecundate one another. In humid environments their reproduction is more intense and they proliferate.

It is estimated that life on earth emerged about 3.5 billion years ago, thus 1 billion years after the formation of the planet. 

Schistosomiasis is a worm infection caused by schistosomes, a species of flatworms (platyhelminthes). The disease is prevalent in Latin America and in the Far East. The main species of schistosome found in Latin America is Schistosoma mansoni. 

In chordates the nervous system is dorsal and highly cephalized while in most invertebrates the nervous system is ganglial and ventral.

Digestion is the breaking down of larger organic molecules obtained from the diet, e.g. carbohydrates, fats, proteins, into smaller ones, like glucose, fatty acids, glycerol and amino acids.

The two main ions that participate in the electrical impulse transmission in neurons are the sodium cation (Na+) and the potassium cation (K+).

The lasting form in pteridophytes is the diploid (2n) sporophyte (the fern itself, for example). In bryophytes the lasting form is the gametophyte (n).

Schistosomes are dioecious, i.e., the species has separated sexes, male and female individuals.

Amphibians, partially aquatic partially terrestrial animals, were the first chordates that colonized the dry land. They came from the aquatic habitat and were originated from fishes. (Nevertheless the first completely terrestrial chordates were the reptiles).

A variety of specialized cells and tissues appeared with extracellular digestion to provide enzymes and special structures for the breaking down of dietary macromolecules. This phenomenon allowed other cells to be liberated for other tasks and differentiations while benefiting from nutrients distributed through the circulation.

As in most cells the region just outside the surface of the neuron plasma membrane presents a positive electrical charge in relation to the region just inside that thus is negative. The normal (at rest) potential difference across the neuron membrane is about – 70 mV (millivolts). This voltage is called the resting potential of the neuron.

Most pteridophytes have subterraneous stems parallel to the substrate called rhizomes. Xaxim is a type of pteridophyte with an aerial stem generally perpendicular to the soil and from which hundreds of roots arise to absorb water from the environment. The xaxim stem is used to make flower pots and other plant supports for gardening (also popularly known as xaxim).

The spontaneous generation hypothesys, or abiogenesis, asserts that life on earth has come from nonliving material. For example, the fact that with time rats appeared around waste was considered in the past a confirmation of this hypothesis. Some supporters of spontaneous generation associated it with the existence of an active principle (the vital elan) that would be the source of life, a theory known as vitalism.

Reproduction in beings of the phylum Chordata is sexual, with the exception of urochordates that can also reproduce asexually. In some classes (cyclostomes, osteichthyes fishes and amphibians) there is larval stage. With rare exceptions, fishes, amphibians, reptiles and monotreme mammals are oviparous, egg laying (embryos develop within eggs and outside the mother’s body) and marsupial and placental mammals are viviparous (embryos develop inside the mother’s body, feeding from her).

Intracellular digestion is that in which the breaking down of macromolecules takes place within the cell. Extracellular digestion is that in which macromolecules are broken down in places outside the cell (in the extracellular space, in the surrounds, in the lumen of digestive tubes, etc.) The advent of extracellular digestion in evolution allowed organisms to benefit from a greater variety of food. The breaking down of larger molecules into smaller ones outside the cell permitted the use of other foods than those that, due the size of their molecules, could not be interiorized by diffusion, phagocytosis or pinocytosis.

This group of plants can be divided into conifers (pine, sequoia, cypress), that have flowers known as strobiles (cones), cycads (very ancient gymnosperms, like the cycads used in garden architecture, that also form strobiles), gnetaceae (gnetum) and ginkgos (the known species is Ginkgo biloba).

The autotrophic hypothesis on the origin of life asserts that the first living beings on earth were producers of their own food, just like plants and chemosynthetic microorganisms.

Urochordates (or tunicates) and cephalochordates are the two subphyla into which protochordates are divided.

Ascidians, sessile animals similar to sponges, are examples of tunicates. The amphioxus, well-studied in Embryology, is an example of cephalochordate.

A variety of specialized cells and tissues appeared with extracellular digestion to provide enzymes and special structures for the breaking down of dietary macromolecules. This phenomenon allowed other cells to be liberated for other tasks and differentiations while benefiting from nutrients distributed through the circulation.

These plants are the typical vegetation of cold regions like the taiga, or boreal forest, of the northern hemisphere, or the araucaria forests of the southern hemisphere.

To refute the spontaneous generation hypothesis many experiments were performed. Francisco Redi, in 1668, verified that maggots appeared on meat only when there was exposition to the environment; within closed environments, they did not appear. In 1862, Louis Pasteur working with swan- neck flasks refuted the abiogenesis hypothesis definitively. In this experiment Pasteur demonstrated that boiled (to kill microorganisms) nutritive soups put in swan-neck flasks (with a curved down mouth so microorganisms could not enter easily) did not contaminate with microorganisms while the same soups within flasks with open upwards mouths were contaminated in a few days. The fact that both flasks were open refuted the argument of the vitalists that the vital elan could not enter the flasks. Pasteur broke the swan-necks of the flasks to demonstrate that proliferation of microorganism could happen if these beings were able to reach the broth. 

The main measures to prevent schistosomiasis are: information for infected individuals to look for treatment and to not disseminate the disease; chemical and biological combat against the vector snail; basic sanitary conditions to avoid contamination of lakes, rivers and other water streams by infected feces; to avoid contact with fresh water suspected of contamination. 

Dichotomy in each of the six following criteria builds the vertebrate evolutionary branch: absence of mandibles separates cyclostomes from others; absence of limbs separates fishes from the remaining; absence of osseous skeleton separates chondrichthian (cartilaginous) fishes from osteichthyes; absence of impermeable skin separates amphibians from the terrestrial vertebrates; absence of warm blood (homeothermic body) separates reptiles from birds and mammals; absence of mammary glands and hair separates birds from mammals.

Animals with an incomplete digestive system are those in which the digestive tube has only one opening (cnidarians, platyhelminthes). Animals with a complete digestive system are those in which the digestive tube has two openings, mouth and anus (all other animal phyla, with the exception of poriferans, that do not have any digestive tube). In animals with incomplete digestive tubes the digestion is mixed, it begins in the extracellular space and finishes in the intracellular space. In animals with complete digestive systems extracellular digestion within the digestive tube predominates.

The primary cause of the neuronal depolarization is the binding of neurotransmitters released in the synapse (by the axon of the neuron that sent the signal) to specific receptors in the membrane of the neuron that is receiving the stimulus. The binding of neurotransmitters to those receptors is a reversible phenomenon that alters the membrane permeability of the region since the binding causes sodium channels to open. When positive sodium ions enter the cell in favor of their concentration gradient, the membrane voltage increases, thus lessening the negative polarization. If this depolarization reaches the excitation threshold (about –50 mV) the depolarization continues, the action potential is reached and the impulse is transmitted along the cell membrane.

Gymnosperms are not cryptogamic as bryophytes and pteridophytes are. They are phanerogamic and so they form flowers and seeds.

Panspermia is a hypothesis that describes life on earth as not originated from the planet. The idea is that the first living beings that colonized the earth came from outer space, from other planets or even from other galaxies by traveling in meteorites, comets, etc. According to this hypothesis even the type of life now existent on earth could have also been seeded intentionally by extraterrestrial beings in other stellar and planetary systems.

Taenias, also know as tapeworms, are platyhelminth animals (flatworms). The main diseases caused by taenias are taeniasis and cysticercosis. 

Bacteria reproduce by binary fission (scissiparity). Some bacteria however present a kind of sexual reproduction  (transformation, transduction or conjugation) with a combination of genetic material from different individuals.

The complete digestive tube allows animals to continuously feed themselves without waiting for residuals to be eliminated before beginning the digestion of new foods. In this way the absorption of larger amount of nutrients is possible and therefore bigger and more complex species can develop. Digestive tubes with two openings also make digestion more efficient since they provide different sites with different physical and chemical conditions (mouth, stomach, bowels) for the action of different complementary digestive enzymatic systems.

The excitation threshold of a neuron is the depolarization level that must be caused by a stimulus to be transmitted as a neural impulse. This value is about –50 mV.

The transmission of the neural impulse along the neuronal membrane obeys an all-or-nothing rule: or it happens with maximum intensity or nothing happens. Always and only when the excitation threshold is reached the depolarization continues and the membrane reaches its maximum possible positive polarization, about +35 mV. If the excitation threshold is not reached nothing happens.

As all plants they present a diplobiontic life cycle, i.e., alternation of generations with diploid and haploid stages. The lasting (final) stage is the diploid one.

According to the heterotrophic hypothesis the first living beings were very simple heterotrophic organisms, i.e., not producers of their own food, which emerged from the gradual association of organic molecules into small organized structures (the coacervates). The first organic molecules in their turn would have appeared from substances of the earth's primitive atmosphere submitted to strong electrical discharges, to solar radiation and to high temperatures. 

Taenias are monoecious (hermaphrodite), the same individual has female and male reproductive organs and undergoes self fecundation. 

Hypothesis to be tested: Vertebrates evolved from echinoderms. Testing material: A newly found protochordate species (i.e., a vertebrate forerunner).

Example of weakening observation (that puts the new protochordate species evolutionarily far from echinoderms): “The new species is protostome”, as opposed to echinoderms, that are deuterostomes. This observation brings the suspicion that deuterostomy in vertebrates is independent in origin from echinoderms.

Example of strengthening observation (that nears the new protochordate species to echinoderms): “The new species has secondary radial symmetry”, similar to echinoderms. This observation makes stronger the hypothesis that echinoderms and vertebrates are relatives.

(This item shows how science works, beginning with the placement of hypothesis and further observational testing.)

Chemical digestion is the series of enzymatic reactions to break macromolecules into smaller ones.

Action potential is the maximum positive voltage level achieved by the neuron in the process of neuronal activation, around + 35 mV. The action potential triggers the depolarization of the neighboring regions of the plasma membrane and thus the propagation of the impulse along the neuron. Resting potential is the membrane voltage when the cell is not excited, about –70 mV. Excitation threshold is the voltage level, about –50 mV, that the initial depolarization must reach for the action potential to be attained.

Pollen grains are the male gametophytes of the phanerogamic (flowering) plants. Therefore within the pollen grains the male gametes of these plants are formed by mitosis.

The present atmosphere of the earth is constituted mainly of molecular nitrogen (N₂) and molecular oxygen (O₂). Nitrogen is the most abundant gas, approximately 80% of the total volume. Oxygen makes up about 20%. Other gases exist in the atmosphere in a low percentage. (Of great concern is the increase in the amount of carbon dioxide due to human activity, the cause of the threatening global warming.) 

Schistosomiasis has acute and chronic phases. Days after the infection the cercarial dermatitis appears at the site where the worm penetrated the skin. In one or two months the acute phase begins and fever, cough, muscle pain and nausea occur but soon disappear; in some people this phase may not manifest. In the chronic phase of the disease the worm infestation can affect the bowels, causing diarrhea, bloody feces, tiredness and hepatosplenomegaly (enlargement of the liver and spleen). The excessive pressure upon the hepatic portal vein produces varix (collateral circulation) in the esophagus and the patient may have hemorrhages and bloody vomiting that may even cause death.

Sexual reproduction occurs when bacteria incorporate genetic material into other bacteria of the same species; the inserted genetic fragment then becomes part of the genetic material of the second bacteria. This kind of reproduction can happen by means of transformation, transduction or conjugation.

Mechanical digestion is the fragmentation of food aided by specialized physical structures, such as teeth, previous to extracellullar digestion. The mechanical fragmentation of food helps digestive enzymatic reactions because it provides a larger total area for the contact between enzymes and their substrates. In some molluscs, the mechanical fragmentation is done by the radula (a teeth-like structure). Some arthropods, like lobsters and dragonflies, have mouthparts that make mechanical digestion of food. In earthworms and birds, the mechanical digestion is made by an internal muscular organ. In mandibulate vertebrates there are mandibles and chewing muscles to triturate food previous to the chemical digestion.

Repolarization is the return of the membrane potential from the action potential (+35 mV) to the resting potential (-70 mV).

When the membrane reaches its action potential voltage-gated sodium channels close and voltage-gated potassium channels open. So sodium stops entering into the cell and potassium starts to exit. Therefore the repolarization is due to exiting of potassium cations from the cell.

The repolarization causes the potential difference temporarily to increase under  -70 mV,below the resting potential, a phenomenon known as hyperpolarization.

The evolutionary importance of the seed is related to the plant capability of distant colonization and to the protection of the embryo. Embryo- containing seeds can be carried by water, wind and animals and germinate in different environments. This fact contributes to the exploration of a variety of ecological niches and for the diversity of plant species. Seeds in addition protect the plant embryo against external aggressions and they also provide germination under more adequate conditions (inside the seed). These features contribute to the evolutionary success of the phanerogam.

The two mains species of taenias that cause disease in humans are Taenia solium, or the pork tapeworm, and Taenia saginata, or the beef tapeworm.

Besides these species there are still the fish tapeworm, or Diphyllobothrium latum, and a tapeworm, Echinococcus granulosus, which lives in dogs and cause in humans the severe disease known as hydatid disease, or echinococcosis. 

Examples of representing beings: protochordates (ascidians, amphioxus), vertebrates. Basic morphology: branchial clefts, notochord, neural tube. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete, deuterostomes. Respiratory system: branchial (in aquatic), cutaneous (in adult amphibians), pulmonary (others). Circulatory system: open in protochordates, closed in vertebrates. Excretory system: diffusion and flame cells in protochordates, kidneys in vertebrates. Nervous system: neural tube in embryos, cerebral vesicle and single ganglion in protochordates, brain within the cranium and spinal cord within the spinal column in vertebrates. Types of reproduction: sexual, with or without larval stage.

The reactions of the extracellular digestion are hydrolysis reactions, i.e., breaking of molecules with the help of water. The enzymes that participate in digestion are hydrolytic enzymes.

The neural impulse is transmitted along the neuronal membrane through depolarization of consecutive neighboring regions. When a region in the internal surface of the membrane is depolarized it becomes more positive in relation to the neighboring internal region. So positive electrical charges (ions) move towards this more negative region and voltage-gated sodium channels are activated and open. The action potential then linearly propagates along the membrane until near the presynaptic region of the axon.

In the male strobiles (cones) there are microsporophylls (fertile leaves of the strobile) that contain microsporangia. Within the microsporagium spore mother cells undergo meiosis and generate haploid microspores. The microspore wall develops into winglike projections (to facilitate its aerial propagation) and mitosis occurs producing two cells, the generative cell and the tube cell. The final structure containing these elements is the pollen grain. The pollen grain relates to the alternation of generations since it is the male gametophyte (the haploid stage of the life cycle).

The earth's primitive atmosphere was basically formed of methane, hydrogen, ammonia and water vapor.

The intermediate hosts of Taenia solium are pigs and the intermediate hosts of Taenia saginata are cattle. 

The protist kingdom includes protozoans and algae. (Two groups of fungi with similar characteristics to protozoans, myxomycetes and oomycetes, have been classified as protists.)  Unicellular protozoans and algae are unicellular eukaryotes. The pluricellular algae are eukaryotes of simple structure too. It is believed that protists are phylogenetic ancestors of living beings of the other eukariotic kingdoms (fungi, animals and plants).

The digestive system, also known as “systema digestorium”, or gastrointestinal system, is composed of the digestive tube organs plus the digestive adnexal glands. The digestive tube is composed of mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum) and anus.

The structure through which the neural impulse passes from one cell to another is the synapse. The synapse is composed by the presynaptic membrane in the terminal portion of the axon of the transmitter cell, the synaptic cleft (or synaptic space) and the postsynaptic membrane in the dendrite of the receptor cell.

In the female strobiles (cones) there are megasporophylls (fertile leaves of the strobile) that contain megasporangia. In the megasporangium the spore mother cell undergoes meiosis generating four haploid cells of which three regress and one gives birth to the functional megaspore. The functional megaspore by several mitosis forms the female gametophyte that contains the oospheres (female gametes) of the plant. The female gametophyte is located within the megasporangium that has a small opening, the micropyle, through which the pollen tube enters. The process is related to the diplobiontic life cycle (alternation of generations) since the functional megaspore generates the haploid stage of the plant (the female gametophyte).

3.5 billion years ago the water cycle was faster than today, resulting in hard storms with intense electrical discharges. There was also no chemical protection from the ozone layer against ultraviolet radiation. The temperatures in the atmosphere and on the planet surface were very high. Electricity, radiation and heat constituted large available energy sources.

Tapeworms have hooks and sucking structures on their heads (scolex) that fixate the parasite in the gut wall; these structures often do not injure the host tissue. The parasite obtains food and makes gas exchange through absorption and diffusion across its skin; since it is a platyhelminth it does not have a digestive system or a circulatory system. 

Fishes are divided into two main classes: chondrichthyes, fishes with cartilaginous skeletons (sharks, rays, dog-fishes), and osteichthyes, bony fishes (tuna, sardines, salmons).

Peristalsis is the process of synchronized contractions of the muscular wall of the digestive tube. Peristaltic movements may occur from the esophagus until and including the bowels. The peristaltic movements are involuntary and they have the function of moving and mixing food along the digestive tube. Peristaltic movement deficiency, for example, in case of injuries of the innervation of the muscular wall of the digestive tube caused by Chagas’ disease, can lead to the interruption of the food traffic inside the bowels and to severe clinical consequences like megacolon (abnormal enlargement of the colon) and megaesophagus (enlargement of the esophagus).

The propagation of the action potential along the axon reaches the region immediately anterior to the presynaptic membrane causing its permeability to calcium ions to change and these ions to enter the cell. In the presynaptic area of the axon there are many neurotransmitter-repleted vesicles that by means of exocytosis activated by the calcium influx release the neurotransmitters into the synaptic cleft. The neurotransmitters then bind to specific receptors of the postsynaptic membrane. (The binding of neurotransmitters to their receptors is reversible, i.e., the neurotransmitters are not consumed after the process.) With the binding of neurotransmitters to the postsynaptic receptors the permeability of the postsynaptic membrane is altered and the depolarization that will lead to the first action potential of the postsynaptic cell begins.

Phospholipids are amphipathic molecules, i.e., they present a polar portion and a nonpolar portion. In contact with water these molecules tend to spontaneously unite and organize themselves forming membranes that create a closed interior space separated from the exterior environment.

Polypeptide chains in their turn can attract water (by electrical attraction) forming a surrounding water layer and also creating an organized structure with delimited interior space. 

Pregnant proglottids with taenia eggs are released together with human feces. If ingested by the intermediate hosts, swine or bovine, the eggs break inside their intestines and the larva trespass the mucosa and gains the circulation to settle on muscles, heart, brain and other organs of these animals and then differentiate into cystic larvae called cysticerci. Humans become infected when eating raw or badly cooked swine or bovine meat contaminated by cysticerci. In the human intestines the cysticerci develop into adult worms and the cycle goes on. 

Compared to cyclostomes, evolutionary novelties presented by fishes are: pectoral and pelvic fins, symmetric and paired; the presence of mandibles.

From the internal surface to the external surface, the digestive tube wall is made of mucosa (epithelial tissue responsible for the intestinal absorption), submucosa (connective tissue beneath the mucous membrane and where blood and lymphatic vessels and neural fibers are located), muscle layers (smooth muscle tissue, two layers, one interior circular and other exterior longitudinal, structures responsible for the peristaltic movement), serous membrane (associated epithelial and connective tissue forming the external surface of the organ). In the bowels the serous membrane prolongs to form the mesentery, a serosa that encloses blood vessels and supports the bowels within the abdominal cavity.

The following are some neurotransmitters: adrenaline (epinephrine), noradrenaline (norepinephrine), acetylcholine, dopamine, serotonin, histamine, gaba (gamma aminobutyric acid), glycine, aspartate, nitric oxide.

Their seeds are formed from differentiation of the megasporangia in the female strobiles (cones). After an oosphere is fecundated the female gametophyte originates the haploid (n) primary endosperm (nutritive tissue that covers the embryo) and the covering of the megasporangium turns into the diploid (2n) seed shell.

Coarcervates are small structures made of the aggregation of organic molecules under water solution. By electrical attraction the molecules join into bigger and more organized particles distinct from the fluid environment forming a membrane-like structure that separates an internal region of the coacervate from the exterior. The coacervates might divide themselves and also absorb and excrete substances. It is believed that these structures may have been the precursors of cells.

The body of the tapeworm is made of segments called proglottids. The proglottids are reproductive structures of the taenia and contain the organs that produce male and female gametes. 

As the proglottids become distant from the scolex (head) they mature. Mature proglottids can fecundate themselves or neighboring ones and the eggs formed are stored inside them. Proglottids called pregnant proglottids, full of eggs, detach from the body of the worm and are eliminated with the human feces. 

“Chondros” means cartilage, “ictis” means fish (both from the Greek); the name chondrichtians is for fishes with cartilaginous endoskeleton. The name osteichthyes comes from the existence of a bony endoskeleton in these fishes (“osteo” means bone, from the Greek too).

There are 6 major salivary glands and they are located one in each parotid gland, two beneath the mandibles (submandibular) and two in the base of the tongue (sublingual). More than 700 other minor salivary glands exist dispersed on the lip mucosa, gingiva, palate and pharynx.

Fluoxetine is a substance that inhibits the re-uptake of serotonin, a neurotransmitter that acts mainly in the central nervous system. By inhibiting the re-uptake of the neurotransmitter the drug increases its availability in the synaptic cleft thus improving the neuronal transmission.

Flowering plants have flowers and seeds (phanerogamic plants). They differ from gymnosperms by having their seeds within fruits.

Coacervates probably provided a nitid separation between an internal and an external environment and thus the organic material within was not lost to the ocean. The enzymatic action inside that internal environment could develop in different manners increasing the speed of specific chemical reactions.

Coacervates also allowed the molecular flux across its membrane to be selective. Since containing different molecules and differently organized from each other, coacervates could have promoted a competition for  molecules from the environment setting out an evolutionary selection.

Pregnant proglottids with taenia eggs are released together with human feces. If ingested by the intermediate hosts, swine or bovine, the eggs break inside their intestines and the larva trespass the mucosa and gains the circulation to settle on muscles, heart, brain and other organs of these animals and then differentiate into cystic larvae called cysticerci. Humans become infected when eating raw or badly cooked swine or bovine meat contaminated by cysticerci. In the human intestines the cysticerci develop into adult worms and the cycle goes on. 

Protozoans are unicellular beings with some similar characteristics to animal cells.

In comparison to pluricellular organisms protozoans are more proximal to the animal kingdom than to plants: they are heterotrophs, they have a rudimentary locomotion system (amoeboid movements, cilia, flagella), they do not have cell wall, some species present structures that resemble structures of a primitive digestive system, with cytostome (mouth) and cytopyge (anus), specialized in digestion and excretion.

The evolutionary hypothesis that animal cells have come from differentiation of protozoans is strong.

The saliva pH is approximately 6.8. It is thus a slightly acid pH. Saliva lubricates the food bolus and initiates the enzymatic extracellular digestion of food. It also works as a  buffer for the mouth pH and it has an important role of having IgA antibodies (also present in tears, colostrum, mother’s milk and in the mucosae of the intestine and airways) that protect the organism against pathogens.

Since the binding of neurotransmitters to the postsynaptic receptors is reversible, after these neurochemicals perform their role they must be eliminated from the synaptic cleft. Neurotransmitters can then bind to specific proteins that carry them back to the axon they came from in a process  called neurotransmitter re-uptake. They can also be destroyed by specific enzymes, like acetylcholinesterase, an enzyme that destroys acetylcholine. Or they can simply diffuse out of the synaptic cleft.

Angiosperm plants are divided into monocotyledonous (monocots) and dicotyledonous (dicots).

It is accepted that the internal membranous organelles of the eukaryotic, like the Golgi apparatus and the endoplasmic reticulum, appeared from invaginations of the external membrane of primitive cells. 

Taeniasis is the parasitic disease caused by the adult tapeworm installed within the human intestine. Cysticercosis occurs when humans ingest eggs or pregnant proglottids of taenias, for example, through badly washed food or self infection. In cysticercosis humans assume the role of intermediate hosts of the parasite and the cysticerci develop within human organs like muscles, brain, eyes and subcutaneous tissue. The brain infestation by cysticerci, a condition known as neurocysticercosis, is extremely severe and may lead to death.

In the normal life cycle of taenias the humans are the definitive hosts and develop taeniasis, a less serious disease, and not cysticercosis. 

Bony fishes have a specialized organ called a gas bladder, or swim bladder, whose interior can be filled with gas liberated from gas glands. The swim bladder works as a hydrostatic organ since it varies the relative density of the body regulating buoyancy and the depth of the animal in water.

Chondrichthyes do not have swim bladders and thus they must continuously agitate their body to keep swimming and maintain their depth in water. As an additional swimming aid, in chondrichthyes the liver is big and oily; this feature helps to reduce their body density relative to water.

The salivary hydrolase is known as salivary amylase, or ptyalin. Ptyalin digests carbohydrates breaking starch and glycogen, glucose polymers, into maltose (a glucose disaccharide) and dextrin.

Neuromuscular synapse is the structure through which the neural impulse passes from the axon of a motor neuron to the muscle cell. This structure is also known as neuromuscular junction, or motor end plate. As in the nervous synapse, the axonal terminal membrane releases the neurotransmitter acetylcholine in the cleft between the two cells. Acetylcholine binds to specific receptors of the muscle membrane, dependent sodium channels then open and the depolarization of the muscle membrane begins. The impulse is then transmitted to the sarcoplasmic reticulum that releases calcium ions into the sarcomeres of the myofibrils thus triggering contraction.

Androecium is the set of male reproductive structures of flowers. It comprehends the stamens formed of filament and anther; one flower has one androecium that may have one or several stamens. Gynoecium is the set of female reproductive structures of flowers. It generally is composed of a single pistil that includes the stigma, the style and the ovary. The androecium usually surrounds the central gynoecium. Besides the androecium and the gynoecium typical flowers are also made of peduncle, sepals and petals.

According to the most accepted hypothesis aerobic eukaryotic cells emerged from the association of aerobic prokaryotes engulfed by primitive anaerobic eukaryotic cells. This would have been the origin of mitochondria that thus would have primitively been aerobic bacteria engulfed by eukaryotic anaerobes. This hypothesis is called the endosymbiotic hypothesis on the origin of mitochondria.

Chloroplasts would also have appeared by endosymbiosis from the entry of photosynthetic prokaryotes into aerobic eukaryotes, both establishing a mutualist ecological interaction.

The main prophylactic measures against taeniasis and cysticercosis are: not to ingest raw or badly cooked swine or bovine meat; sanitary education of the people; appropriate destination of feces; adequate treatment of infected people. 

Fishes are all aquatic animals and thus they have a hydrodynamic elongated body suitable to move under water, without limbs and with fins. The habitat conditions the branchial respiration too.

When food is swallowed the swallow reflex is activated and the larynx elevates and closes to avoid portions of the food bolus entering the trachea causing aspiration of strange material to the bronchi.

Information about the conditions of the external and internal environments, like temperature, pressure, touch, spatial position, pH, metabolite levels (oxygen, carbon dioxide, etc.), light, sounds, etc., are collected by specific neural structures (each for each type of information) called sensory receptors.

Sensory receptors are distributed throughout the tissues according to their specific roles. The receptors get that information and transmit them through their own axons or through dendrites of neurons that connect to them. The information reaches the central nervous system that interprets and uses it to control and regulate the body. 

The main differentiation criteria between monocots and dicots are: number of cotyledons (seed leaf) in seeds, one in monocots and two in dicots; pattern of leaf veins, parallel in monocots, reticulated in dicots; multiplicity of petal number, multiples of three in monocots, multiples of four or five in dicots; position of vascular bundles in the stem, scattered in monocots, concentrically ringed in dicots.

Grasses, banana tree, sugar cane, orchids are examples of monocots. Sunflowers, oaks and waterlilies are examples of dicots.

The fact that chloroplasts are the organelles responsible for photosynthesis in plants leads to the supposition that before symbiosis they were autotrophic prokaryotes. For the reason that mitochondria are the center of the aerobic cellular respiration, the powerhouse of the eukaryotic cell, it is supposed that they were once aerobic prokaryotes.

The endosymbiotic hypothesis to explain the emergence of aerobic and autotrophic eukaryotic beings is strengthened further by the following evidence: chloroplasts as well as mitochondria have their own DNA, similar to bacterial DNA; chloroplasts and mitochondria reproduce asexually by binary division, like bacteria do; both organelles have ribosomes and synthesize proteins. 

Ascaris, or Ascaris lumbricoides, is an animal of the nematode phylum, i.e., a roundworm. Ascaris causes ascariasis, a common worm infection of the intestine.

Protozoans are eukaryotic cells so they have organelles and structures common to this kind of cell: endoplasmic reticula, Golgi apparatus, digestive vesicles, ribosomes, mitochondria, nucleus with genetic material, karyotheca, etc. All these elements are found dispersed throughout the cytoplasm. Protozoans do not have cell walls.

Protozoans from the mastigophora group (like trichomonas) have flagella and others, others from the ciliated group (like paramecium) have cilia.

The esophagus is a predominantly muscular organ so the assertion is correct. The esophagus is a muscular tube formed in its superior third of striated muscle tissue, in its middle third of mixed muscle tissue (striated and smooth) and in its lower third of smooth muscle tissue. The peristalsis of the esophagus provides the movement of the food towards the stomach even without gravitational help.

Sensory receptors are structures specialized in the acquiring of information, like temperature, mechanical pressure, pH, chemical environment and luminosity, transmitting them to the central nervous system. Sensory receptors may be specialized cells, e.g., the photoreceptors of the retina, or specialized interstitial structures, for example the vibration receptors of the skin. In this last case they transmit information to dendrites of sensory neurons connected to them. There are also sensory receptors that are specialized terminations of neuronal dendrites (e.g., the olfactory receptors).

The heterotrophic hypothesis asserts that the first living beings were the fermenting heterotrophs.  Fermentation released carbon dioxide (CO₂) and then the atmosphere became enriched by this gas. By mutation and natural selection organisms capable of using carbon dioxide and light to synthesize organic material appeared. These would have been the first photosynthetic beings (that were also fermenting beings since there was no abundance of oxygen).

Since photosynthesis is a reaction that releases molecular oxygen, with the emergence of fermenting autotrophs this gas became available. Some organisms then developed aerobic respiration using O₂, a highly efficient method to produce energy.

If a person eats raw or badly cooked meat infected by Taenia solium or Taenia saginata he or she will develop the disease taeniasis. The ingestion of contaminated raw or badly cooked infected meat means that cysticerci are being ingested. The human then will be affected by taeniases since in his or her gut the cysticerci may develop into an adult tapeworm. 

From Hydrostatics it is known that an object does not sink if its density is equal or less than the density of the fluid in which it is immersed.

When the swim bladder is filled by gas it reduces the density of the fish body and when it is emptied this density is increased. So this mechanism controls the fish depth under water.

Until reaching the duodenum the food enters the mouth, passes the pharynx, goes down the esophagus and passes the stomach.

The sensory receptors are classified according to the stimuli they get: mechanoreceptors are stimulated by pressure (e.g., touch or sound); chemoreceptors respond to chemical stimuli (olfactory, taste, pH, metabolite concentration, etc.); thermoreceptors are sensitive to temperature changes; photoreceptors are stimulated by light; nocireceptors send pain information; proprioceptors are sensitive to the spatial position of muscles and joints (they generate information for the equilibrium of the body).

The pollen tube that is the mature male gametophyte of angiosperms has three cellular nuclei: two sperm nuclei and one tube cell nucleus.

All those nuclei are part of the male gametophyte of the plant and thus each of them is haploid (n).

Ultraviolet radiation from the sun was not disallowed to reach the surface of the primitive earth. Therefore the development of life on dry land or even near the aquatic surface was impracticable. Probably the first living beings lived submerged in deep water to avoid destruction by solar radiation. Only after the appearance of photosynthetic beings and the later filling of the atmosphere with oxygen released by them the formation of the atmospheric ozone layer that filters ultraviolet radiation was possible.

The ascaris live within the human gut and feed from the food ingested by the infected person. 

As in every vertebrate the circulatory system of fishes is closed, i.e., blood flows only within blood vessels.

The fish heart has only two consecutive chambers: a thin-walled atrium and a muscular ventricle. The arterial (oxygenated) blood comes from the gills and gains arteries towards tissues, then venous blood is collected by veins and reaches the atrium of the heart passing to the ventricle that pumps the venous blood towards the gills to be again oxygenated.

The valve that separates the stomach from the esophagus is the cardia. It has the function of preventing acid gastric content from entering back into the esophagus. Insufficiency of this valve causes gastroesophageal reflux, a disease in which patients complain of bloating and heartburn (retrosternal burning).

Vision is the ability of some living beings to perceive, to distinguish and to interpret luminous stimuli. Vision is important on earth mainly in the terrestrial and in the superficial aquatic habitats because our planet is intensely exposed to sunlight and thus light and colors become distinguishing factors of objects present in the environment, even at distance. This distinction provided new survival strategies for the organisms, new protection mechanisms against external dangers, new ways to find food and to communicate with other individuals, new types of courting and reproduction behaviors, etc. That is, it created new possibilities of interaction with the surrounds and increased capacity to explore new ecological niches.

In the flower ovary there are megasporangia enclosed by a tegument having a small opening, the micropyle. Within the megasporangium there is a megasporocyte, or megaspore mother cell, that undergoes meiosis forming four megaspores of which three regress and only one is functional. The functional megaspore undergoes (three) mitosis generating eight cells that as a whole form the embryonic sac.

The problem that the theory of evolution, or simply evolution, and its rival theories try to solve is to explain how the different living beings that live on earth have appeared.

In the pulmonary phase the ascaris infestation causes cough, hemoptysis, dyspnea, fever, fatigue and may cause a special kind of pneumonia called eosinophilic pneumonia. In the intestinal phase the symptoms are due to spoliation of nutrients of the host and thus hunger and weight loss may appear. Masses of ascaris inside the bowels can cause severe intestinal obstruction. 

Fishes “breath” through gills. Gills, or branchiae, are highly vascularized organs specialized in gas exchange under water and present in aquatic animals (marine annelids, crustaceans, fishes and tadpoles). Gills are a respiratory organ (analogous, for example, to lungs) containing very thin lamellae with many apparent blood vessels in direct contact with water.

In osteichthyes the gills are covered by a bony flap that protects them called operculum. In chondrichthyes there are no opercula.

The valve that separates the stomach from the duodenum is the pylorus. It has the function of keeping the food bolus within the gastric cavity for enough time to allow the gastric digestion to take place. It also has the function of preventing the intestinal content from going back into the stomach. 

In the first mentioned group of animals there are photoreceptor cells organized in ocelli or diffusely dispersed in the body. These animals do not form images. In the animals of the second group the photoreceptor cells are part of more sophisticated structures, the eyes, able to form images and to send them to the nervous system.

The main theory that opposes the evolution theory on the explanation of how species emerged (phylogenesis) is fixism. 

Ascaris is a monoxenous parasite, its life cycle is dependent only on one host and so it does not have intermediate host. 

All protozoans, as eukaryotes, have nucleus. Some species, like the paramecium, have two nuclei: the macronucleus and the micronucleus.

The normal pH of the gastric juice is around 2. So it is an acid pH. It is necessary for the gastric pH to be kept acid for the activation of pepsinogen (a proenzyme secreted by the gastric chief cells) into pepsin, the digestive enzyme that acts only under low pH. This pH level is attained by the secretion of hydrochloric acid (HCl) by the parietal cells.

The organs of the human visual apparatus are the eyes, the optical nerves and the visual areas of the brain (located in the occipital lobes of both hemispheres).

The embryonic sac is the female gametophyte of angiosperms. The embryonic sac is composed of three cells that remain next to the micropyle, two lateral synergids and the central oosphere (egg); one binucleated cell, the polar nuclei, is placed in the central region; three antipodal cells stay in the opposite side to the micropyle

Since all these cells come by mitosis from the functional megaspore they are haploid (n).

The main prophylactic measures against ascariasis are: efficient washing of vegetables and other foods; basic sanitary conditions and appropriate destination of feces; hygiene education for people; combat against insects that can carry the eggs of the parasite, like flies and cockroaches. 

Fishes have a pair of kidneys that filtrate the blood. Bony fishes excrete nitrogen as ammonia, NH₃, (they are ammoniotelic) and cartilaginous fishes excrete urea as nitrogen waste (they are ureotelic, like adult amphibians and mammals).

With its corrosive effect, HCl also helps the rupture of the adhesion between food particles, facilitating the digestive process.

The main structures of the human eye are the cornea, the iris, the pupil, the ciliary muscles, the crystalline lens and the retina (the space between the crystalline lens and the retina within the eyeball is filled with vitreous humor).

Self pollination occurs when pollen grains from a flowering plant fall into the pistils of the same plant and thus gametes from the same individual unite to form a zygote. Cross pollination occurs when pollinators carry pollen grains from a plant to reach other individual plants of the same species thus gametes of different individuals form the zygote.

Since it promotes formation of zygotes containing genes from different individuals (new gene combinations) cross pollination contributes more to biological diversity.

The main arguments in favor of evolutionism are: paleontological, from the study of similarities among fossils of different periods; of compared anatomy, the existence of structures with same origin and function and of residual organs, like the human appendix, that reveal relationships among species; of compared embryology, similarities of structures and developmental processes among embryos of related species; of molecular biology, larger percentage of similar nucleotide sequences in the DNA of species having common ancestors. 

Adult ascaris that live within the human intestine can release up to 200 thousand eggs a day. The eggs are eliminated with human feces and mature in the environment under some heat and moisture conditions. Humans may ingest mature eggs through food contaminated by human feces or through bad hygienic habits. The eggs again inside the human intestine release larvae that cross the enteric mucosa and gain the circulation reaching the lungs. In the lungs the larva mature and go to the airway and to the pharynx when they are then swallowed. Within the gut the larvae develop into adult worms. 

The macronucleus is properly the cell nucleus, it has DNA and RNA and acts as the center of the cellular control and regulation. The micronucleus has reproductive functions and it is related to the conjugation process (sexual reproduction).

The gastric epithelium is mucus secretory, i.e., it produces mucus. The mucus covers the stomach wall preventing corrosion by the gastric juice.

The iris works like the diaphragm of a photographic camera since it has muscles that contract or relax varying the pupil diameter. When the luminous intensity heightens the parasympathetic nervous system commands the contraction of the pupil; when there is shortage of light the sympathetic nervous system stimulates the dilation of the pupils. These movements depend upon the muscles of the iris.

After pollination one of the sperm nuclei from the pollen tube unites with the oosphere of the embryonic sac forming the diploid (2n) zygote. The other sperm nucleus fuses with the polar nuclei of the embryonic sac originating a triploid (3n) cell that by mitosis will turn into the secondary endosperm of the seed. The synergids and the antipodal cells degenerate after the fecundation process. Fecundation in these plants is indepednet from water.

Fossils are petrified vestiges of beings that lived in the past conserved by chemical and geological processes and found within rocks and sedimentary strata of the terrestrial crust. 

Ancylostomiasis is a disease caused by Ancylostoma duodenale or Necator americanus, both hookworms belonging to the nematode phylum (roundworms). Ancylostomiasis caused by these worms is also called hookworm disease.

Since the parasites nourish themselves on human blood the infection causes anemia, hypoproteinemia and the patient often seems pale. 

Protozoans are heterotroph beings, i.e., they do not make their own food and thus they need to search for it in the environment. Protozans have developed several locomotion mechanisms and they actively move towards food.

The digestive enzyme that acts in the stomach is pepsin. Pepsin has the function of breaking proteins into smaller peptides. The gastric cells that produce pepsinogen (the zymogen precursor of pepsin) are the chief cells.

The crystalline is a converging spherical lens. This natural lens has the function to project images of objects onto the retina.

Dichogamy is the phenomenon of the maturation of female reproductive structures of the plant in a different period to the maturation of the male reproductive structures. Dichogamy prevents self pollination and makes cross pollination almost obligatory so assisting in an evolutionary strategy to promote genetic recombination.

The study of fossils reveals ancient and extinct species having many similar structures to others of the present and of the past. Fossils still allow radioactive dating to estimate the periods during which species lived and to establish a chronological relationship between them. Those evidences strengthen the hypothesis of relationship and common origin among species and that their features have modified gradually until the formation of the current species. 

The main prophylactic measures against hookworm disease are: to avoid walking barefoot on soils suspected of contamination; basic sanitary conditions and appropriate destination of feces; treatment of infected people. 

The lateral lines of bony fishes are sense organs that extend along both sides of the animal body. They make contact with the environment by a series of specialized scales that transmit information about pressure variation and vibrations in the surrounding water.

The partially digested and semifluid food bolus that leaves the stomach and enters the duodenum is called chyme.

The photoreceptor cells form the retina, a lamina that covers the internal posterior region of the eyeball. The photosensitive cells of the retina are divided into two types: the cone cells and the rod cells. These cells have pigments that sense specific light wave ranges (frequencies) and trigger action potentials conducted by the optical nerves to the visual area of the brain.

A typical seed is composed of the embryo, endosperm and shell. Within seeds of angiosperms there are one or two cotyledons (seed leaf, one in monocots, two in dicots). The endosperm is the tissue within the seed that has the function of nourishing the embryo.

The two main evolutionary theories were lamarckism and darwinism.

Both Ancylostoma duodenale and Necator americanus have mouthparts with hooks or “teeth” that help the fixation of the parasite in the human intestine wall and facilitate the tissue injury necessary to drain blood from the host. The structures are evolutionary adaptations for the parasitic way of life of these animals. 

In chondrichthyes fecundation is internal by means of copulation. In osteichthyes fecundation generally is external and the gametes are released in the water, where they can fecundate their counterpart and form the zygote.

The small intestine is divided into three portions: duodenum, jejunum and ileum.

Visual accommodation is the phenomenon of varying the curvature of the crystalline lens to make possible the variation of its refractivity to adjust the images of objects exactly onto the retina. The visual accommodation is accomplished by the action of the ciliary muscles.

The nitid vision depends on the visual accommodation since, if the images are not projected onto the retina but in front or behind it, they will appear blurred. The closer an object is more the ciliary muscles must compress the crystalline lens (increasing its curvature); the more distant an object is more the ciliary muscles must relax.

Fruits are modified ovaries of the flowers.

According to the law of use and disuse the characteristics of a body vary as it is more or less used. This rule is valid for example for features like the muscular mass and the size of the bones.

The law of the transmission of acquired characteristics in its turn established that parents could transmit to their offspring characteristics acquired by the law of use and disuse.

Hookworms are monoxenous, i.e., their life cycle depends only on one host.

Amoebae move by amoeboid movements, small projections and invaginations of their plasma membrane (pseudopods) that alter the external morphology of the cell making it move on surfaces. Paramecia have the outer face of their plasma membrane covered by cilia that flap helping the cell to  move. Trichomonas are flagellated protozoans, i.e., they have relatively long filaments outside the cell that beat and make possible active swimming in fluid environments.

Until the exit of the stomach, carbohydrates, in the mouth, and proteins, in the stomach, have already undergone chemical breaking by digestive enzymes. Carbohydrates have suffered action of the salivary amylase (ptyalin) and proteins have suffered action of the enzyme pepsin of the gastric juice. Fats, until reaching the duodenum, do not undergo chemical digestion.

Since the crystalline lens is a convex spherical lens it forms inverted images on the retina (every converging lens forms inverted images). The inverted information follows through the optical nerves until the occipital cerebral cortex that contains the visual area of the brain. In the brain the interpretation of the image takes place and the inverted information is reverted.

The fecundation in angiosperms triggers the release of hormones that act upon the ovaries. The ovary wall then develops into a fruit that contains the seeds.

Adult hookworms within the human intestine release eggs that are eliminated with the human feces. Under adequate conditions of moisture and temperature the eggs mature in the soil and generate larvae. The larvae differentiate into thread-like infective larvae that can penetrate the human skin, generally through the feet. The larvae them gain the human circulation and reach the lungs from where they go to the airway and the pharynx. When the larvae are swallowed they enter the small intestine and develop into adult worms and the cycle restarts. 

Examples of representing beings: sharks, rays, sardines, tuna, salmons. Basic morphology: hydrodynamic body, fins; cartilaginous skeleton in chondrichthyes, bony skeleton in osteichthyes. Skin: with scales in osteichthyes and placoid scales in chondrichthyes. Respiration: branchial. Circulation: closed, incomplete, heart with two chambers. Nitrogen waste: urea in chondrichthyes, ammonia in osteichthyes. Thermal control: heterothermic. Types of reproduction: sexual, internal fecundation in chondrichthyes, external in osteichthyes.

Bile, an emulsifier liquid, is made by the liver and later stored within the  gallbladder and released in the duodenum. Bile is composed of bile salts, cholesterol and bile pigments. Bile salts are detergents, amphiphilic molecules, i.e., molecules with a polar water- soluble portion and a non-polar fat- soluble portion. This feature allows bile salts to enclose fats inside water-soluble micelles in a process called emulsification for them to be in contact with intestinal lipases, enzymes that break fats into simpler fatty acids and glycerol.

The near point is the closest distance between an object and the eye that makes possible the formed image to be focused, i.e., it is the point in which the ciliary muscles are in their maximum contraction. The far point is the most distant point from the eye in which an object can be placed and its image is still focused, i.e., it is the situation of maximum relaxation of the ciliary muscles. The zone between the near point and the far point is called the accommodation zone.

In gymnosperms the endosperm is haploid (n), it is called primary endosperm. In angiosperm the endosperm is triploid (3n), it is called secondary endosperm.

Charles Darwin was an English naturalist born in 1809 and considered the father of the theory of evolution. At the end of the year 1831, before turning 23 years of age, Darwin embarked as volunteer scientist on the ship the Beagle for a five year expedition to the South American coast and the Pacific. During the voyage, whose most famous passage was the stop in the Galapagos Islands, Darwin collected data that he used to write his masterpiece “The Origin of Species” (1859). In this book the principles of the common ancestry of all living beings and of natural selection as the force that drives the diversity of species were described. Darwin died in 1882.

(The original name of the most famous book written by Darwin was “On the Origin of Species by Means of Natural Selection”.)

A prion is an infectious (transmissible) protein able to replicate by transforming other proteins into a copy of the prion. The mechanism of copying is not yet understood by science. The hypothesis come out from research about a nervous system disease known as Creutzfeldt-Jacob disease, epidemiologically associated to a bovine disease called bovine spongiform encephalitis (the mad cow disease).

Research discovered that the infectious agent that causes those diseases, suprisingly, was a protein capable of copying itself and of being transmitted by ingestion (the reason why meat from contaminated animals cannot be consumed), inoculation and even heredity. 

 Digestion in protozoans is intracellular digestion: organic material is internalized and degraded inside the cell.  Protozoans get food by phagocytosis and then the food is digested when phagosomes fuse with lisosomes within the cell, forming digestive vacuoles. The digestive vacuoles give origin to residual bodies that are eliminated from the cell by exocytosis.  In the paramecium the entrance of food into the cell and the excretion of digestive residuals occur at specialized regions of the plasma membrane, the cytostom and the cytopyge, respectively.

Bile is concentrated and stored in the gallbladder. When fat rich foods are ingested the gallbladder contracts to release bile inside the duodenum. (This is the reason why patients with gallstones must not ingest fatty food, the reactive contraction of the gallbladder may move some of the stones to the point of blocking the duct that drains bile into the duodenum, causing pain and possible severe complications.)

Presbyopia is the visual impairment in which there is loss of the cililary muscle strength thus reducing the capability of the crystalline lens to adjust images of near objects onto the retina. In presbyopia the near point of vision becomes more distant. The disease generally occurs in old people.

Astigmatism is caused by irregular shape of the refractive structures, mainly the cornea. In astigmatism a single object-point may produce more than one image onto the retina and so the vision becomes distorted.

In some so-called fruits the actual fruit is not the flesh part. For example, the flesh part of the strawberry is not the fruit. The fruits are the small hard dots on the surface of the strawberry. Another example: the flesh part of the cashew is not the fruit. The fruit is the nut. The edible part of the onion is the stem of the plant and not the fruit.

The mentioned mechanism is the natural selection.

The main known human diseases of such type are the Creutzfeldt-Jacob disease (CJD), the kuru and the Gerstmann-Sträussle-Scheinken disease (GSS). The hypothesis that many other diseases of unknown etiological agents are actually caused by self-replicating infectious proteins is strong. 

The amphibians are totally aquatic in the larval stage and partially terrestrial animals as adults and for these facts they are considered intermediate beings in the evolutionary passage of vertebrates from the aquatic to the dry land habitat. Amphibians are also the first tetrapod animals, i.e., the first with two pair of limbs, a typical feature of terrestrial vertebrates. The name “amphibian” comes from the double life (aquatic as larvae and partially terrestrial as adults) of these animals.

Besides making bile for release in the duodenum, the liver has other digestive functions. The venous network that absorbs nutrients from the guts, called mesenteric circulation, drains its blood content almost entirely to the hepatic portal vein. This vein irrigates the liver with absorbed material from the digestion. So the liver has the functions of storing, processing and inactivating nutrients. Glucose is polymerized into glycogen in the liver; this organ also stores many vitamins and the iron absorbed in the intestine. Some important metabolic molecules, like albumin and clotting factors, are made in the liver from amino acids of the diet. In the liver ingested toxic substances, like alcohol and drugs, are inactivated too.

Myopia is the visual condition in which the images are formed before (in front of) the retina. Hypermetropia is the visual condition in which the point of image formation is beyond (behind) the retina. Actually myopia is due to an increase in the distance between the retina and the crystalline lens, mainly caused by a slight flattening of the eyeball. In hypermetropia the retina is too close to the crystalline lens due to slight shortening of the eyeball.

In myopia the near point and the far point of vision come closer (the refractivity of the crystalline lens that corresponds to the maximum distension capacity of the ciliary muscles is not enough to provide visual accommodation). In hypermetropia the ciliary muscles are not able to contract more to compensate the inadequate position of the retina, i.e., the near point becomes more distant.

Cotyledons, or seed leaves, are structures formed by the embryo of angiosperms to absorb nutrients from the endosperm and to store and transfer these nutrients to the embryo. (Cotyledons are auxiliary embryonic structures).

Seeds of monocots have a single cotyledon. Seeds of dicots have two cotyledons.

Degenerative diseases are non infectious prevalent diseases whose incidences increase with aging. 

Permeable skin, body subject to dehydration, external fecundation, eggs without shells and larval stage with branchial respiration are features that make amphibians dependent on water to survive.

The other adnexal gland of the digestive system is the pancreas. This organ makes digestive enzymes that digest proteins (proteases), lipids (lipases) and carbohydrates (pancreatic amylases). Other digestive enzymes, like gelatinase, elastase, carboxipeptidase, ribonuclease and deoxyribonuclease are also secreted by the pancreas.

The middle ear is formed by the tympanum, the ossicular chain and the oval window. The functional ossicles of the middle ear are the hammer (malleus), the incus and the stapes.

The main functions of fruits are the protection and spreading of seeds.

Darwin recognized that in a same species there were individuals with different characteristics. He also realized that those differences could lead to different survival and reproduction chances for each individual. Therefore he discovered the importance of the environment acting upon organisms and preserving those having more advantageous characteristics for survival and more able to generate offspring and so he described the basis of the principle of natural selection.

Hypertension is a disease in which the arterial blood pressure, during systole or during diastole, is abnormally high. Hypertension, or high blood pressure, is a condition that must be diagnosed and treated since it produces irreversible injuries in arteries and, later, it causes other severe diseases in organs like the heart, brain, kidneys, retina, etc. 

In amphibians the embryonic development is indirect (there is a larval stage).

The pancreas secretes trypsinogen that, undergoing action of the enzyme enterokinase secreted by the duodenum, is transformed into trypsin. Trypsin in its turn catalyzes the activation of pancreatic chymotrypsinogen into chymotrypsin. Trypsin and chymotrypsin are proteases that break proteins into smaller peptides. The smaller peptides are then broken into amino acids by the enzyme carboxipeptidase (also secreted by the pancreas in a zymogen form and activated by trypsin) helped by the enzyme aminopeptidase made in the intestinal mucous membrane.

The structures of the human auditory sensitivity are the ears (external, middle and internal), the  vestibulocochlear nerves and the auditory areas of the brain (located in the temporal lobes of both hemispheres).

Plants that produce single-seeded fruits, for example, mango and avocado, often have ovaries with only one egg inside. Fruits with more than one seed are originated from plants whose ovaries have more than one egg.

Both lamarckism and darwinism are evolutionary theories as opposed to fixism, both admit the existence of processes that caused changes in the characteristics of the living beings in the past.

They have however different explanations for those changes. Lamarckism combines the law of use and disuse with the law of the transmission of acquired characteristics to explain the changes. Darwinism defends the action of the natural selection.

Risk factors for a disease are everything that contributes to increase the risk of the disease to appear. For example, for most cardiovascular diseases, tobacco smoking and diabetes mellitus are both important risk factors; for some cancers having a genetic component a positive family history is a risk factor, etc. 

In fishes gas exchange is done by direct contact of water with the branchiae (gills). Gases gain and exit the circulation through the gills.

In adult amphibians gas exchange is done through the moist and permeable skin (cutaneous respiration) and also through the lungs, a set of tiny airway terminations associated to a highly vascularized tissue specialized in gas exchange.

The axolotl is an exotic amphibian found in Mexico that lives in water and “breathes” through gills even as an adult.

Carbohydrate digestion begins with the action of the salivary amylase (ptyalin) in the mouth and it continues in the duodenum by the action of the pancreatic juice. This juice contains the enzyme pancreatic amylase, or amylopsin, that breaks starch (amylum) into maltose (a disaccharide made of two glucose molecules).

The human ear is divided into three mains parts: the external ear, the middle ear and the internal ear.

Infructescences are aggregated fruits formed from inflorescences, aggregated flowers. Grape clusters are examples of infructescences. Pseudofruits are “fruits” not made in the ovaries and in general their true fruits lack development and are found within the flesh, like in apples and pears. Parthenocarpic fruits are those made without fecundation, by means of hormonal stimuli, like bananas.

The modern darwinist theory that incorporates knowledge from Genetics and Molecular Biology is called neodarwinism, or synthetic theory of evolution. 

The main risks factors for hypertension are tobacco smoking, stress, obesity, sedentary lifestyle and alcoholism. 

The larva of amphibians has exclusively branchial respiration. This is one of the reasons why it depends on water to survive.

The enzyme pancreatic lipase is present in the pancreatic juice. This enzyme breaks triacylglycerol (triglyceride) into fatty acids and glycerol.

The tympanum (or ear drum) is a membrane located in the middle ear just after the auditory canal and so it separates the middle ear from the external ear. The function of the tympanum is to vibrate with the same frequency of the sound waves that reach it.

The fruits contain seeds and they can detach from the plant falling on the ground or can serve as food for animals. Therefore with the emergence of fruits the seeds of angiosperms could be transported across long distances contributing to the propagation of the species.

Today it is known that variation of inherited characteristics is created by  alterations in the genetic material of the individuals, more precisely by modifications or recombinations of DNA molecules. Small changes in the genetic material accumulate and new phenotypical characteristics emerge.

The carriers of these characteristics then are submitted to natural selection. From modern Biology its recognized that natural selection generates in a given population an increase in the frequency of alleles and genes more favorable to survival and reproduction; less advantageous genes and alleles tend to be eliminated.

The maximum blood pressure is the pressure on the wall of the systemic arteries during systole, i.e., when the heart is pumping blood to arterial vessels. The minimum blood pressure is the pressure on the wall of the systemic arteries during diastole, i.e., when the heart ventricles are relaxing and getting blood.

The fish heart has only two chambers, an atrium and a ventricle, and the blood that comes to it is purely venous.

In amphibians there are three heart chambers (a second atrium is present) and there is arterial blood coming from the lungs; in these animals the heart has two atria (one that gets blood from the body and other that gets blood from the lungs) and one ventricle; arterial blood mixes with venous blood within the ventricle which in turn pumps the blood to the lungs and to the systemic circulation.

The enteric juice is secreted by the small intestine mucosa. The enzymes of the enteric juice and their respective functions are described as follows:

Enterokinase: enzyme that activates trypsinogen into trypsin. Saccharase: enzyme that breaks sucrose (saccharose) into glucose and fructose. Maltase: enzyme that breaks maltose into two glucose molecules. Lactase: enzyme that breaks lactose into glucose and galactose. Peptidases: enzymes that break oligopeptides into amino acids. Nucleotidases: Enzymes that break nucleotides into its components (nitrogen-containing bases, phosphates and pentoses).

The internal ear comprises the pinna, or auricle, and the auditory canal. Its function is to conduct the sound waves to the tympanum.

A tendency of the gametophyte evolution in plants has been towards the formation of gametes that are independent from water. In bryophytes and in pteridophytes the fecundation is totally dependent on water. In phanerogamic plants such dependency does not exist.

Another tendency is the reduction in the size and duration of the gametophyte. In bryophytes the gametophyte is indeed the lasting stage. In pteridophytes, gymnosperms and angiosperms it became the temporary stage and its relative size was successively reduced. A third evolutionary trend relates to the interdependency between gametophytes and sporophytes. In bryophytes the sporophyte is entirely dependent on the gametophyte to survive. In the remaining plants the sporophyte is the independent stage and the once autotrophic gametophyte in bryophytes and pteridophytes became dependent upon the sporophyte in the phanerogamic plants.

The main human degenerative diseases are divided into three groups: cardiovascular diseases, neoplastic diseases and degenerative diseases of the nervous system. The main cardiovascular diseases are hypertension, the cardiopathies, including coronary disease and myocardial infarction, and the cerebrovascular accidents (CVAs, or strokes). Neoplasias are benign tumors and cancers. The main degenerative diseases of the nervous system are Alzheimer's disease and Parkinson's disease.

Adult amphibians have kidneys that filter blood. Nitrogen waste is excreted as urea (so amphibians are ureotelic beings). The larvae, aquatic, excrete ammonia.

Entering the duodenum the chyme meets the pancreatic juice under a pH of approximately 8.5. The neutralization of the chyme acidity is necessary to keep adequate pH level for the functioning of the digestive enzymes that act in the duodenum. Without the neutralization of the chyme acidity the mucous membrane of the intestine would be injured. When stimulated by the chyme acidity the duodenum makes a hormone called secretin. Secretin stimulates the pancreas to release the pancreatic juice and also the gallbladder to expel bile in the duodenum. The pancreatic secretion, rich in bicarbonate ions, is released in the duodenum and neutralizes the chyme acidity; this acidity is also neutralized by the secretion of bile in the duodenal lumen.

The acoustic transmission from the external to the middle ear (and to the internal ear too) is entirely mechanical. The vibration of the tympanic membrane triggers the vibration of the hammer that then causes the incus to vibrate. The incus then causes the stapes to vibrate.

Plant tissues are divided into growth (embryonic) tissues, supporting, filling and photosynthetic tissues  (ground tissues), conductive (vascular) tissues and covering (dermal) tissues.

Embryonic tissues: primary meristems; secondary meristems. Supporting tissues: collenchyma; sclerenchyma. Filling and photosynthetic tissues: photosynthetic parenchyma; storage parenchyma. Conductive tussues: xylem; phloem. Covering tissues: epidermis; periderm.

Living beings are considered under reproductive isolation when they cannot cross among themselves or if they can cross but cannot generate fertile offspring. 

The main degenerative diseases of the heart are heart failure, arrhythmias, valvular heart diseases, coronary insufficiency and myocardial infarction. 

Fresh water is the less concentrated of solutes than sea water and it (fresh water) tends to be less concentrated than the intracellular environment making cells to swell. Sea water, on the other hand, since it is very concentrated tends to dehydrate the cell.

The vacuoles of protozoans are internal structures specialized in water storage that when necessary liberate water to the cytoplasm. Vacuoles thus can dilute the cytoplasm for it to enter into osmotic equilibrium with the environment. Protozoans of fresh water then need vacuoles more since their intracellular is hypertonic in relation to the exterior. Without the dilution mechanism provided by the vacuoles, protozoans of fresh water would absorb too much water and would die.

The human digestive secretions are: saliva, gastric juice, bile, pancreatic juice and enteric juice. Among these  secretions only the bile does not contain digestive enzymes.

In the internal ear there are the cochlea and the semicircular canals. The fluid that fills the cochlea receives vibration from the ossicular chain of the middle ear and transmits the pressure to the semicircular canals. Within the semicircular canals the pressure variation of the filling fluid moves cilia of the hair cells of these structures. The hair cells then generate action potentials that are transmitted to the brain through the auditory nerves.

Lateral buds are portions of meristematic tissue located in the base of the shoots. Apical buds are portions of meristematic tissue situated in the tip of the stem and shoots.

Reproductive isolation is an important concept because it defines the concept of species: only living beings that can cross among themselves and generate fertile offspring, i.e., that are not under reproductive isolation, belong to the same species. For example, humans and chimps are under reproductive isolation and are not of the same species. 

Myocardial infarction is the condition in which an area of this tissue or the entire heart muscle dies by hypoxia due to lack of blood irrigation. Myocardial infarction is a severe disease since on the dependence of its extension the heart can fail, i.e., it can no longer pump blood to the lungs or to the body or it can even stop beating (causing death). 

The main cause of myocardial infarction is coronary obstruction, blocking of the arteries that carry arterial blood to the heart muscle. Other events like hemodynamic shock (circulation stoppage due to large hemorrhages, for example) can also cause myocardial infarction. 

In the majority of the amphibian species fecundation is external. This feature is common to bony fishes too and it shows that the reproductive system and the embryonic development of amphibians are a heritage form osteichthyes.

Curiously although having external fecundation amphibian male and female copulate to stimulate the liberation of sperm and egg cells. This phenomenon does not characterize internal fecundation since the gametes unite in water.

The stomach and the pancreas make zymogens of the proteases pepsin, chymotrypsin and trypsin and these zymogens are released into the gastric or duodenal lumen for activation. This happens to prevent the digestion of these organs' (stomach and pancreas) own cells and tissues by the active form of the enzymes. So the production of zymogens is a protective strategy against the natural effects of the proteolytic enzymes.

The vestibular system is the part of the ear that participates in the control and regulation of the equilibrium of the body (balance).

The semicircular canals of the inner (internal) ear are perpendicularly placed and detect changes in the gravitational position of the head (this is another sensorial function of the inner ear, besides auditory perception). When the head rotates the pressure of the fluid within the canals upon the cilia of specific receptor cells varies and these cells generate action potentials transmitted by the vestibulocochlear nerve. The neural impulse is then interpreted by the brain as information about the gravitational position of the head.

Apical meristems are those primary meristems found in the apex of the stem and in the tips of shoots and roots. The apical meristems are responsible for the primary growth of the plants.

The four main groups of protozoans are the sarcodines (that form pseudopods, like amoebae), the mastigophores (flagellated, like the trypanosome that causes Chagas’ disease), the ciliated (like paramecia) and the sporozoans (spore-forming, like plasmodia).

Evolution tried to solve this problem in two ways. The simplest is the long and tubular shape of the bowels (approximately eight meters in extension), making possible that numerous small intestine loops fold closely. More efficient solutions are the intestinal villi and the microvilli of the mucosal membrane cells. The intestinal wall is not smooth. The mucous membrane, together with its submucosa, projects inside the gut lumen like glove fingers forming invaginations and villi that multiply the available surface for absorption. In addition the epithelial cells that cover these villi have themselves numerous hairlike projections called microvilli on the external face (lumen face) of their plasma membrane. The absorptive area of the intestines is thus increased hundreds of times with these solutions. In the jejunum and ileum there are folds that have the function of increasing the absorption surface too.

The pressure inside the middle ear is maintained equal to the external ear (so to the exterior too) due to a communicating duct between the middle ear and the pharynx called the auditory tube, or Eustachian tube. When someone goes down a mountain the air pressure upon the middle ear increases and it is necessary to do some exercises like fake swallowing to force the opening of the pharyngeal orifice of the auditory tube to equalize the pressure again.

The growth tissues of the plants are the meristems. Meristems are the tissues that produce the plant growth giving birth to all other tissues; they are formed of undifferentiated cells having intense cell division rate. Meristems classify as primary meristems and as secondary meristems.

Primary meristems are found in the apex of the stem, in the lateral buds of the stem, in the basis and tips of the shoots and within the root cap. Primary meristems are responsible for the primary growth (lengthening) of the plant.

Secondary meristems are those that make the plant grow in thickness (secondary growth) and they are formed by tissues that thicken the stem: cambium and phellogen (cork cambium).

Coronary artery bypass graft is a kind of surgical myocardial revascularization, i.e., a way to provide blood to a myocardium whose blood supply is impaired or blocked due to coronary disease. In this surgical treatment of coronary disease one or more blood vessel grafts taken from other parts of the body are used to join the obstructed artery (in a region after the obstruction) with the aorta or other healthy coronary artery thus reestablishing the blood flow to the myocardium. Often the blood vessel grafts are part of the saphenous vein from the leg or of the mammary artery from the chest or even of the radial artery from the forearm. 

Eyelids associated to lacrimal glands protect and keep eyes lubricated against damage from the great luminosity of terrestrial environments. Fishes do not have eyelids since their eyes are in constant contact with the fluid medium.

Most part of water, vitamins and mineral ions are absorbed by the small intestine. The large intestine, however, is responsible for the reabsorption of nearly 10% of the ingested water, an important amount that gives consistency to feces (colon diseases can cause diarrhea).

Endocrine gland is a gland whose secretions (called hormones) are collected by the blood and reach the tissues through the circulation. The hypophysis (pituitary) and the adrenals are examples of endocrine glands. Exocrine gland is a gland whose secretions are released externally through ducts (into the skin, intestinal lumen, mouth, etc.). The sebaceous glands and the salivary glands are examples of exocrine glands.

Lateral, or secondary, meristems, are the cambium and the phellogen (also known as cork cambium), tissues from the stem, branches and roots that by mitosis generate other tissues. These tissues participate in the secondary growth of plant, i.e., in the thickening of the stem, branches and roots. 

Adaptive convergence is the phenomenon by which living beings facing the same environmental pressure (problems) and undergoing genetic variability and natural selection incorporate similar (analogous) organs and structures (solutions) into their bodies during evolution. For example, the fins and the hydrodynamic body of fishes and dolphins, phylogenetically distant animals. 

Cerebrovascular accident (CVA), also known as stroke, is the generic name given to infarction (tissue and cellular death by hypoxia) of areas of the brain due to vascular obstruction or hemorrhages. CVAs are divided into ischemic and hemorrhagic. In the ischemic CVA blocking of arteries that carry blood to the brain occurs; its cause is generally atherosclerosis (atheroma formation) of these vessels. In the hemorrhagic CVA there is rupture of one or more blood vessels of the brain with blood leakage, increasing intracranial pressure and thus interruption of blood flow in some areas of the brain. The severity of the stroke depends on the function performed by the affected area of the brain, for example, motor function, visual function, vegetative function, etc., and on the size of the involved area. 

The main risk factors for cerebrovascular accidents are hypertension, hypercholesterolemia, tobacco smoking and old age. 

Euglenas are involved in taxonomic polemics because they tend to be classified sometimes as protozoans and sometimes as algae. Although they have chloroplasts and they are photosynthetic  autotrophic beings, euglenas do not have a cell wall and they can survive by “eating” substances from the environment when light is not available for photosynthesis. Curiously euglenas also have a photosensitive structure called stigma that orients the movement of the cell towards light. Nowadays euglenas are classified as algae, but it is suspected that they are common ancestors of algae and protozoans.

Monosaccharides, amino acids, mineral salts and water are absorbed by the intestinal epithelium and collected by capillary vessels of the intestinal villi. From the capillaries, nutrients go to the mesenteric circulation, a system of vessels that drains the intestinal loops. The blood of the mesenteric circulation is drained to the portal hepatic vein and some nutrients are processed by the liver. From the liver, nutrients are gathered by the hepatic veins that discharge its blood content into the inferior vena cava. Blood from the inferior vena cava then gains the right chambers of the heart and is pumped to the lungs for oxygenation. From the lungs the blood then returns to the heart where it is pumped to the tissues distributing nutrients and oxygen.

The endocrine system is said to have integrative character since the hormones produced by the endocrine glands are substances that act at a distance and many of them act in different organs of the body. So the endocrine glands receive information from some regions of the body and they can produce effects in other regions providing functional integration for the body.

Besides the endocrine system, the other physiological system that also has integrative function is the nervous system. The nervous system integrates the body through a network of nerves connected to central and peripheral neurons. The endocrine system integrates the body through hormones that travel through the circulation and are produced by the endocrine glands.

Meristematic cells have very thin cell walls, small vacuoles, a well-centralized nucleus and they are constantly undergoing mitosis. Meristematic cells need a high mitotic rate because they are responsible for the plant growth.

Geographical isolation between groups of the same species leads to formation of a new species since it disallows crossing among isolated individuals. Distinct characteristics from the other groups are incorporated by genetic variability and natural selection into the isolated groups until the emergence of a new species. So the geographical isolation creates the reproductive isolation. 

Not every tumor is neoplastic and not every neoplasia creates tumor. Tumor is the generic name given to the abnormal increase in mass or volume of any area of the body (for example, the enlarged tonsils during throat infection are a kind of tumor, any inflammation creating a swelled area characterize a tumor, etc.). Neoplasias can form tumors, some of them very large, by aggregation of neoplastic cells in the region where the neoplasia began or in distant implantations. Cancer is a synonym for malign neoplasia.  

The main problems vertebrates coming from water needed to solve to adapt to the terrestrial environment were the following: the problem to avoid dehydration; the problem of elimination of wastes in a medium where water is less available; the problem of protection against nocent solar radiation; the problem of gamete locomotion in the  environment for fecundation; the problem of gas exchange, earlier done by direct contact of water with gills; the problem of body support, since it was water that played this role in fishes.

Solutions for the dehydration problem: thicker and impermeable skin, to lose less water, or moist and permeable skin, like in amphibians. Solution for the excretion problem: excretion of urea (also excreted by chondrichthyes) or uric acid, substances that need less water to be dissolved. Solutions for the problem of protection against radiation: skin pigments that filter harmful radiation, feathers, hair or carapaces.

Solution for the gamete movement problem: internal fecundation (except for most amphibians, that have external fecundation). Solution for the gas exchange problem: appearing of airways and lungs. Solution for the body support problem: further development of muscular and bony structures, like limbs and claws.

The endocrine system is constituted by the endocrine glands and the hormones they secrete.

The best hypothesis is that the tissue is a sample of meristematic tissue. Meristematic tissues seen under the microscope have many cells undergoing mitosis.

Adaptive radiation is the appearance of several other species from one common ancestral species that have spread to various regions or environments. The different characteristics among the species correspond to the adaptive necessities of the ecological niches each one occupies, i.e., to different environmental pressures. 

Neoplasia is any abnormal and uncontrolled proliferation of cells of an organism. Neoplasias can be benign or malign. Benign neoplasias are those in which the cell proliferation is limited to a given site of the body and so neoplastic cells do not spread to other close regions or at distance through the circulation. Malign neoplasias are those in which the neoplastic cells disseminate at distance to other sites and organs of the body, a process called metastasis, where they continue to proliferate. 

Malign neoplasias injure tissues and if not eradicated they are fatal. Benign neoplasia can also be deadly when it forms a tumor that grows and compresses vital organs. 

There are algae that reproduce sexually and there are algae that reproduce asexually.

In unicellular algae reproduction is generally asexual by binary division.

In pluricellular algae asexual reproduction can occur by fragmentation or by sporulation.

In sexual reproduction of algae, uni or pluricellular, there is fusion of gametes (syngamy). There are algae in which all cells can become gametes and there are algae in which only some cells can play that role. Some species may present alternation of generations, forming gametophytes and sporophytes with different ploidies.

Lipoproteins are complexes made of lipids (triglycerides and cholesterol) and proteins. The lipoproteins present different densities according to the relationship between their protein and lipid quantities since lipids are less dense than proteins. Low-density lipoproteins (LDL) are those with a low protein/lipid relation; high-density lipoproteins (HDL) have a high protein/lipid relation; another group is the very low density lipoproteins (VLDL) with very low protein/lipid relation.

LDL is known as “bad cholesterol” because it transports cholesterol from the liver to the tissues and so induces the formation of atheroma plaques inside blood vessels, a condition called atherosclerosis (do not confuse with arteriosclerosis) that can lead to severe circulatory obstructions like acute myocardial infarction, cerebrovascular accidents and thrombosis. HDL is known as “good cholesterol” since it transports cholesterol from the tissues to the liver (to be eliminated with the bile) and elevation of the HDL blood level reduces the risk of atherosclerosis. (VLDL transforms into LDL after losing triglycerides in the blood).

The glands are epithelial tissues. They are made of epithelium that during the embryonic development invaginated into other tissues. In the exocrine glands the invagination has preserved secretion ducts. In the endocrine glands the invagination is complete and there are no secretion ducts.

The plant-filling tissue is generically called parenchyma. The plant parenchyma can be divided into photosynthetic parenchyma, a tissue that has cells with many chloroplasts and a high photosynthesis rate found mainly in leaves, and storage parenchyma, specialized in the storage of water (e.g., in cactus), starch or air (e.g., in aquatic plants).

Fishes and dolphins have similar organs and shape because although they have phylogenetically distant ancestors they face similar environmental pressures since they share the same habitat (water). So by undergoing genetic variability and natural selection some similar features, for example, the hydrodynamic body and the presence of fins, were incorporated into these animals. 

Cancers are malign neoplasias, i.e., abnormal and uncontrolled proliferation of cells that can disseminate to other sites of the body. Cancer dissemination at distance usually occurs through blood or lymphatic vessels. 

Many algae have high nutritional value and are commercialized and consumed as human food, they are very popular  food in the oriental world. Jelly compounds are extracted from some algae, like glues and pastes for industrial and commercial use.

The agar-agar, used as a medium for biological culture in laboratories and in medicines, and the substance known as carrageenin, a component of tooth pastes, cosmetics, paint and hygienic products, are extracted from rhodophyte algae. Diatom algae deposited on the bottom of the sea form diatomites, used in the production of filters, refractories, thermal isolation and cement. Some algae are used as agricultural fertilizers.

Some types of plant fibers are not absorbed by the intestine but play an important role in the functioning of the organ. They retain water inside the bowels and thus contribute to the softening of the fecal bolus. A softer fecal bolus is easier to be eliminated during defecation. People that eat less dietary fiber may suffer from hard stools and constipation.

Hormones are substances secreted by the endocrine glands and collected by the circulation that act to produce effects upon specific organs and tissues. Hormones are effectors of the endocrine system.

The plant supporting tissues are the collenchyma and the sclerenchyma. The collenchyma is made of living and elongated cells that accumulate cellulose and pectin in some regions of the cell wall making them unequally thick and thus providing flexibility. The sclerenchyma is made mostly of dead cells killed by lignin deposition (lignin is an impermeable biopolymer) forming elongated, rigid and impermeable fibers. The sclerenchyma is a plant tissue widely used in the textile industry.

Characteristics of different species are said to be analogous when having the same biological function, for example, the wings of bats and the wings of insects.

Characteristics of different species are said to be homologous when having the same biological origin, i.e., when they are products of differentiation of a same characteristic from a common ancestor, like cat paws and human feet. (Characteristics of different species may be analogous and homologous.)

Neoplasias appear due to DNA mutations in genes that regulate the cellular proliferation thus making the cell lose its capacity to control and limit its division by mitosis. The cell then divides continuously and uncontrollably and this defect is transmitted to its daughter cells. 

Examples of representing beings: frogs, toads, salamanders. Basic morphology: two pairs of limbs, eyelids, hydrodynamic larvae. Skin: moist and permeable, mucous glands. Respiration: cutaneous and pulmonary, branchial in larval stage. Circulation: closed, incomplete, heart with three chambers without interventricular septum.

Nitrogen waste: urea. Thermal control: heterothermic. Types of reproduction: sexual, water dependant, external fecundation and aquatic larval stage.

Bacteria that live inside the gut have great importance in digestion. Some polysaccharides like cellulose, hemicellulose and pectin are not digested by the digestive enzymes secreted by the body, instead, they are broken by enzymes released by bacteria of the gastrointestinal tract. The intestinal bacterial flora also make vital substances for the functioning of the bowels facilitating or blocking the absorption of nutrients and stimulating or reducing peristalsis. Some gut bacteria are the main source of vitamin K for the body and so they are essential for the blood clotting process.

In the intestinal flora there are utile but also potentially harmful bacteria. It is estimated that more than 100 trillion bacteria live in a human gut. Some bacteria are useful too because they compete with other species preventing excessive proliferation of these bacteria.

The main endocrine glands of the human body are the pineal gland (or pineal body), the hypophysis (or pituitary), the thyroid, the parathyroids, the endocrine part of the pancreas, the adrenals and the gonads (testicles or ovaries). Other organs like the kidneys, the heart and the placenta also have endocrine functions.

The main photosynthetic tissue is the photosynthetic parenchyma (also known as chlorenchyma, do not confuse with collenchyma) often located between the  superior and the inferior epidermis of the leaves.

Ecology is the field of Biology that studies the relationship between living beings and between living beings and the environment. 

Excluding skin cancer, that are the more easily detected and so cases are registered in larger number, the main types of cancer in men are prostate cancer, lung cancer, stomach cancer; in women, breast cancer, colon and rectal cancer and lung cancer are of great incidence. Other common cancers are ovarian cancer, pancreatic cancer, liver cancer, esophageal cancer, brain cancer and the leukemias and lymphomas (blood cancers).

Epithelial cancers, of the skin as well as of the internal organs, are more common because epithelial tissues are more exposed to carcinogens. The proportional incidence of the many types of cancers varies according to the considered population. 

Red tide is a phenomenon that occurs when dinoflagellates (algae from the pyrrophyte group) proliferate excessively in the ocean. These algae liberate toxins that affect the nervous system and can cause death when ingested by marine animals and by humans that eat contaminated animals.

The hormones that participate in the regulation of digestion are gastrin, secretin, cholecystokin and enterogastrone.

The circulatory system is fundamental for the functioning of the endocrine system. The blood collects the hormones made by the endocrine glands and through the circulation these hormones reach their targets. Without the circulatory system the 'action at distance' characteristic of the endocrine system would not be possible.

Water, mineral salts and sugar are transported throughout the plant through conductive vessels formed by specialized tissues.

A species is a set of living beings able to cross among themselves generating fertile offspring.

This concept however does not apply to individuals of exclusive asexual reproduction and so other definitions have been proposed. For example, “a species is a set of living beings that evolve in a common manner all of them considered ancestors of the same type in relation to common descendants”. 

Carcinogens are factors capable of producing neoplasias. Any mutagen, a substance that can induce DNA mutation, is a potential carcinogen. Examples of carcinogens are radiation, nitrous acid, many substances inhaled through tobacco smoking and the human papilloma virus (HPV). 

Bacteria are prokaryotic and unicellular beings. Bacteria have simple organization, they present an external cell wall, plasma membrane, circular DNA within the cytoplasm and ribosomes for protein synthesis. Some bacteria are encapsulated, i.e., they have a polysaccharide capsule outside the cell wall.

The first entirely terrrestrial vertebrate class, totally independent from the aquatic habitat, is the class Reptilia.

The presence of food in the stomach stimulates the secretion of gastrin that in its turn triggers the releasing of the gastric juice.

The pineal gland, also known as pineal body or epiphysis, is situated centrally in the head. It secretes the hormone melatonin, a hormone produced at night and related to the regulation of the circadian rhythm (or circadian cycle, the wakefulness-sleep cycle). Melatonin possibly regulates many body functions related to the night-day alternation.

The main cells that form the phloem are the sieve elements and the companion cells. The sieve elements form the vessel walls; they are living enucleated cells positioned in series forming the sieve tubes. Between successive vessel elements there are communicating pores. The companion cells are located outside and alongside the sieve tubes and they help in the absorption of the material to be transported.

A population is a set of individuals of the same species found in a given place in a given time. 

The main risk factor for lung cancer is tobacco smoking. The large number of cases of this type of cancer is due to the increased number of smokers worldwide. 

Prokaryotic beings are classified into two big groups: archaebacteria and bacteria (this last also known as eubacteria).

Compared to bacteria, archaebacteria have basic differences, like the chemical compositions of their plasma membrane and cell wall and different enzymes related to DNA and RNA metabolism.

There are unicellular and pluricellular fungi. All fungi are eukaryotes and heterotrophs.

Fungi have cells with cell wall made of chitin, the same substance that constitutes the exoskeleton of arthropods. Fungi, likewise animals, characterize for storing glucose in the form of its polymer glycogen.

Secretin is made in the duodenum. The chyme acidity causes the duodenum to release this hormone that in its turn stimulates the secretion of the pancreatic juice.

Some hormones are proteins, like insulin, glucagon and ADH, others are derived from proteins (modified amino acids), like adrenaline and noradrenaline, other are steroids, like the corticosteroids and estrogen.

The vascular tissues of the plants are the xylem and the phloem. Xylem is the plant tissue that forms the vessels that conduct water and mineral salts absorbed from the soil to the plant cells. Phloem is the plant tissue that forms the vessels that conduct dissolved sugar from the leaves (where they are produced by photosynthesis) to other plant cells.

A community is a set of populations of living beings that live in the same region and interact with each other.

In Ecology a population is a set whose members (living in a given place in a given time) are part of the same species. A community is a set of populations of different species (living in a given place in a given time).

In neoplastic tumors a phenomenon called angiogenesis occurs. Angiogenesis is the formation of new blood vessels. Neoplastic cells induce the formation of new blood vessels to irrigate and drain the neoplastic tissue. Angiogenesis is important because the tumor growth depends on it. A lot of research on cancer has tried to discover natural and synthetic substances to inhibit angiogenesis. 

There are three peculiar types of archaebacteria. The halophile archaebacteria only survive in salt-rich environments (even salinity of the sea is not enough for them). Thermoacidophile archaebacteria are characterized by  living under high temperatures and low pH. The methanogen archaebacteria are those that liberate methane gas (CH₄), they are found in swamps.

The reptile skin is keratinized and impermeable to water while the amphibian skin is permeable. The skin impermeability made impossible the cutaneous gas exchange performed by amphibians and respiration became dependent on internal organs like airways and lungs.

The fat level of the chyme detected in the duodenum stimulates the secretion of cholecystokin (CCK). CCK acts by stimulating the secretion of the pancreatic juice also, and the releasing of bile by the gallbladder.

The pituitary gland, or hypophysis, is located in the sella turcica of the sphenoid bone (one of the bones in the base of the skull). So the gland is situated within the head.

Vascular cambium is the secondary meristematic tissue that in roots and in the stem forms the vascular tissues (xylem and phloem) of the plant. Usually the outer side of the vascular cambium produces a layer of phloem and the inner (more central) side of the tissue produces a layer of xylem.

An ecological niche is a set of peculiar activities, resources and strategies that a species explores to survive and reproduce. An habitat is the place where the species lives to explore its ecological niche. In other words it can be said that the habitat is the “address” of the species and the ecological niche is the “profession” of the species. 

The main risk factor for skin cancer is solar exposition of the skin without protection against ultraviolet radiation (a potential carcinogen). The most lethal skin cancer is melanoma. 

All fungi are heterotrophs (so, they do not perform photosynthesis). Fungi are typical decomposers, they eat and degrade organic material.

When the chyme is too fatty there is a secretion of enterogastrone by the duodenum. This hormone reduces the peristalsis of the stomach thus slowing the entrance of food into the duodenum (as the digestion of fats takes more time).

The hypophysis is divided into two portions: the adenohypophysis, or anterior hypophysis, and the neurohypophysis, or posterior hypophysis.

In the adenohypophysis two hormones that act directly, the growth hormone (GH) and the prolactin, and four tropic hormones, i.e., hormones that regulate other endocrine glands, the adrenocorticotropic hormone (ACTH), the thyroid-stimulating hormone (TSH), the luteinizing hormone (LH) and the follicle-stimulating hormone (FSH) are produced.

The neurohypohysis stores and releases two hormones produced in the hypothalamus, oxytocin and the antidiuretic hormone (ADH, or vasopressin).

The main cells of the xylem are the tracheids and the vessel elements (these only in angiosperms). The tracheids and the vessel elements are dead cells that have lost their cytoplasm and only their cell wall impregnated with lignin (an impermeable biopolymer) remained. The tracheids form tubes that communicate with neighboring tubes through pores; the vessel elements do not present pores but instead they communicate with the successive vessel element through perforations in their extremities.

Biotic factors are living beings (plants, animals and microorganisms) that are part of a given environment.

If the cancer is in its initial stage treatment is often done by surgical removal of the neoplastic tissue. Cancers already disseminated are often treated with radiation (radiotherapy) and anti-mitotic drugs (chemotherapy). 

Snakes are carnivorous. Iguanas are herbivorous.

The digestive tube of birds has special structures, in this sequential order: the crop, the proventriculus and the gizzard. The crop has the function of temporary storage of ingested food and it is a more dilated area of the avian esophagus. The proventriculus is the chemical stomach of the birds where food is mixed with digestive enzymes. The gizzard is a muscular pouch that serves as a mechanical stomach where the food is ground to increase the exposure area of the food particles for the digestive enzymes to act.

The hypothalamus is a part of the brain situated just above the hypophysis. The hypothalamus gets peripheral and central neural impulses that trigger response of its neurosecretory cells. The axons of these cells go down to the adenohypophysis to regulate the hipophyseal secretions by means of negative feedback. When the plasma levels of adenohypophyseal hormones are too high the hypothalamus detects this information and commands the interruption of the production of the hormone. When the blood level of an adenohypophyseal hormone is low the hypothalamus stimulates the secretion of the hormone.

The hypothalamic cells produce the hormones released by the neurohypophysis. These hormones are transported by their axons to the hypophysis and then released in the circulation.

Vascular bundles are segments of xylem and associated phloem that run longitudinally within the stem. In dicots the vascular bundles are organized side- by-side forming concentric rings. In monocots the vascular bundles are scattered and do not form rings.

Abiotic factors are the nonliving elements that constitute a given environment, like light, temperature, minerals, water, gases, atmospheric pressure, etc. 

Parkinson’s disease is a degenerative disease of the nervous system in which the main manifestations are progressive motor disturbances, like tremors of feet, hands and mandibles (jaws) and walking and balance impairments.

Parkinson’s disease is due to the degeneration of dopaminergic motor neurons, i.e., motor neurons that use dopamine as a neurotransmitter, located in a specific region of the brain, the mesencephalon. Such degeneration creates deficiency of dopamine in the nervous system. (Parkinson’s disease should not be confused with other causes of tremors, like the use of some medicines.) 

Bacteria are used by industry in various ways. There are vaccines made of attenuated pathogenic bacteria or of antigens present in bacteria. One of the most ancient uses of bacteria is the fermentation of milk to produce yogurt, cheese and curd (even before the knowledge of the existence of bacteria these microorganisms were already used in the making of those products). Some methods of antibiotic production involve bacteria. The recombinant DNA technology (genetic engineering) allows the industrial production and commercialization of human proteins, like insulin for diabetics, synthesized by mutant bacteria. Some bacteria can produce fuel, like methane gas.

These beings do not have permeable skin so they do not make cutaneous respiration like amphibians do. Just like in birds and in mammals, their respiration is pulmonary.

Bird feces are more liquid than mammal feces, i.e., less water is absorbed in the avian digestive system. The more frequent elimination of feces in birds due to their less solid feces is an adaptation to flight since their body weight is kept lower.

The adenohypophisys secretes GH (growth hormone), prolactin, ACTH (adrenocorticotropic hormone), TSH (thyroid-stimulating hormone), FSH (follicle-stimulating hormone) and LH (luteinizing hormone).

GH, also known as somatotropic hormone (STH), acts upon bones, cartilages and muscles promoting the growth of these tissues. Prolactin is the hormone that in women stimulates the production and secretion of milk by the mammary glands. The ACTH is the hormone that stimulates the cortical portion of the adrenal gland to produce and secrete the cortical hormones (glucocorticoids). The TSH is the hormone that stimulates the activity of the thyroid gland increasing the production and secretion of its hormones T3 and T4. The FSH is a gonadotropic hormone, i.e., it stimulates the gonads and in women it acts upon the ovaries inducing the growth of follicles, in men it stimulates spermatogenesis. The LH is also a gonadotropic hormone that acts upon the ovaries of women to stimulate ovulation and the formation of the corpus luteum (that secretes estrogen) and in men upon the testicles to stimulate the production of testosterone.

Leaf veins are made of vascular tissues. They are constituted by xylem and phloem that respectively conduct water and mineral nutrients (xylem) and sugar (phloem).

An ecosystem is a system composed of biotic and abiotic factors in interaction.

The main degenerative diseases of the nervous system are Alzheimer’s disease and Parkinson’s disease. Degenerative diseases of the nervous system are caused by progressive tissue degradation or loss of neurons in some regions of the nervous system. 

Pathogenic bacteria have characteristics known as virulence factors that help 

them to parasite their host. Some bacteria have fimbriae, cilium-like structures that attach the bacterial cell to the host tissue. There are bacteria specialized in intracellular parasitism. Other bacteria secrete toxins, molecules that cause disease; in some cases, the bacterial population growth causes food contamination by toxins. Generally, bacterial disease is caused by bacterial population growth with invasion and destruction of tissues or by bacterial toxins that contaminate the organism.

The kingdom fungi is divided into four phyla: ascomycetes, basidiomycetes, zygomycetes and deuteromycetes.

Mushrooms are basidiomycetes. Fungi are heterotrophs and decomposers (they break down dead beings) and they actively participate in the recycling of organic material in ecosystems. Some fungi keep mutualist ecological interaction with algae or cyanobacteria, forming lichen, and with plant roots, forming mycorrhizas.

Herbivorous animals eat great amounts of cellulose, a substance not digested by their digestive enzymes. In these animals regions of the digestive tube are colonized by microorganisms that digest cellulose. This mutualist ecological interaction between animals and microorganisms occurs, e.g., in horses, cows, rabbits and in some insects such as termites.

The rings observed on a stem cross section of dicot trees are made of conductive tissues: xylem and phloem.

A biosphere is a set of all of the ecosystems of the planet.

Alzheimer’s disease is a degenerative disease of the central nervous system in which the patient has progressive dementia and alteration of mental functions.

The disease generally appears after 40 years of age and it is more frequent in the elderly. Image studies of the brain show broad loss of brain tissue. (The Alzheimer’s disease should not be confused with other mental deteriorations common in the elderly.) 

Bacteria can be found in various environments throughout the planet. There are bacteria in the air, in fresh water, on the surface, in the intermediate depth and on the bottom of the sea, in soils, in our skin and practically in all terrestrial environments through which air circulates freely. Some bacteria can be found in volcanic craters under extremely high temperatures.

Fungi are industrially used in the production of fermented beverage, bread, cheese, etc. Some fungi are very important for the production of medical drugs. There are fungi processed to serve as food for humans, like eatable mushrooms.