Amphibians – Characteristic Features

General features

Nervous system The brains of the placental mammals possess a corpus callosum, which is absent in monotremes and marsupials. The elephant is the only mammal known to have no pleural space. Anura grenouilles et crapauds. Certaines espèces sont carnivores au stade têtard, mangeant des insectes, de plus petits têtards et des poissons. Allobates zaparo n'est pas toxique, mais mime l'apparence de deux espèces toxiques qui partagent son aire de répartition pour se prémunir des prédateurs [ ].

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To carry out a radioimmunoassay, an antigen is made radioactive , often with radioactive iodine attached to tyrosine. A measured quantity of this radiolabeled antigen and a known amount of antibody to that antigen are then mixed, and the two bind together. Subsequently, in a separate assay, a serum sample containing an unknown concentration of the same antigen not radiolabeled is added. The unlabeled antigen then competes with the radiolabeled antigen for antibody-binding sites. The proportion of antibody bound to unlabeled antigen can then be determined.

Used for scraping, it has minute chitinous teeth. Such deviations can be quite large in small populations. Rats transmit a wide variety of diseases to human beings. RBC Red blood corpuscle. A hybrid cross occurring between the same two types of organisms, but with the sexes of the parents reversed. It connects the sigmoid colon with the anal canal. During the replication of DNA the two strands of a duplex DNA molecule are first unzipped by helicase and topoisomerase enzymes to form a replication fork see figure right , then each of the separated "parental" strands is used as a template for the synthesis of a new strand.

Single-strand binding proteins bind the separated strands to prevent them from re-annealing. On the leading parental strand see figure DNA polymerase proceeds continuously producing the new leading strand. Because the lagging replication complex see figure proceeds away from the fork, it produces the new lagging strand in a piecewise fashion; it must wait for the replication fork to open further before each of the new pieces known as "Okazawi fragments" after their discoverer R.

For example, the human reproductive cycle involves two forms, a haploid gamete alternating with a diploid organism. Each form produces the other. Reptiles differ from amphibians in having an amnion , a feature held in common with mammals and birds. RER Rough endoplasmic reticulum. They cut the internal phosphodiester bonds of the molecule. In the living organism, energy is liberated, along with carbon dioxide, through the oxidation of molecules containing carbon.

The term respiration denotes the exchange of the respiratory gases oxygen and carbon dioxide between the organism and the medium in which it lives and between the cells of the body and the tissue fluid that bathes them.

With the exception of energy used by animal life in the deep ocean, all energy used by animals is ultimately derived from the energy of sunlight. The carbon dioxide in the atmosphere in conjunction with the energy of sunlight is used by plants to synthesize sugars and other components. Animals consume plants or other organic material to obtain chemical compounds , which are then oxidized to sustain vital processes. This article considers the gaseous components of air and water, the natural respiratory habitats of animals, and the basic types of respiratory structures that facilitate gas exchange in these environments.

Although the acquisition of oxygen and the elimination of carbon dioxide are essential requirements for all animals, the rate and amount of gaseous exchange vary according to the kind of animal and its state of activity. In the Table the oxygen consumption of various animals is expressed in terms of millilitres of oxygen per kilogram of body weight per hour, reflecting the gas demands of different species at rest and in motion.

A change in the chemical composition of the body fluids elicits a response from the central nervous system , which then excites or depresses the machinery of external respiration. The form of the lungs and the methods of irrigating them may also influence activity by affecting the efficiency of gas exchange.

In snakes the lungs are simple saclike structures having small pockets, or alveoli, in the walls. In the lungs of all….

The range of respiratory problems faced by aquatic and terrestrial animals can be seen from the varying composition and physical characteristics of water and air. Air contains about 20 times the amount of oxygen found in air-saturated water. In order to extract an equivalent amount of oxygen as an air breather, an aquatic animal may find it necessary to pass across the respiratory surfaces a relatively larger volume of the external medium.

Moreover, the diffusion rate of oxygen is much lower in water than in air. The problem is further compounded by the higher density 1, times air and viscosity times air of water, which impose on the machinery of aquatic respiration a much greater work load. Thus, fish may expend about 20 percent of their total oxygen consumption in running the respiratory pump, as compared with about 1 to 2 percent in mammals, including humans.

The carbon dioxide content of most natural waters is low compared with air, often almost nil. In contrast to oxygen, carbon dioxide is extremely soluble in water and diffuses rapidly. Most of the carbon dioxide entering water combines either with the water to form carbonic acid or with other substances to form carbonates or bicarbonates.

This buffering capacity maintains a low level of free carbon dioxide and facilitates the maintenance of a favourable diffusion gradient for carbon dioxide exchange by water breathers. In general, oxygen exchange, which is strongly dependent on the oxygen content of the water, is more critically limiting for aquatic forms than is the exchange of carbon dioxide.

Temperature exerts a profound effect on the solubility of gases in water. At the same time, a rise in body temperature produces an increase in oxygen consumption among animals that do not closely regulate their body temperatures so-called cold-blooded animals. A fish experiencing both rising water and body temperatures is under a double handicap: The amount of oxygen available in natural waters is also limited by the amount of dissolved salts.

This factor is a determinant of oxygen availability in transitional zones between sea and fresh water. Bodies of water may have oxygen-poor zones. Such zones are especially evident in swamps and at the lower levels of deep lakes. Many animals are excluded from such zones; others have become remarkably adapted to living in them. It is composed of a mixture of gases held in an envelope around the globe by gravitational attraction.

The atmosphere exerts a pressure proportional to the weight of a column of air above the surface of the Earth extending to the limit of the atmosphere: Dry air is composed chiefly of nitrogen and inert gases These percentages are relatively constant to about At sea level and a barometric pressure of millimetres of mercury, the partial pressure of nitrogen is The existence of water vapour in a gas mixture reduces the partial pressures of the other component gases but does not alter the total pressure of the mixture.

To calculate the partial pressures of the respiratory gases, this value must be subtracted from the atmospheric pressure. Atmospheric pressures fall at higher altitudes, but the composition of the atmosphere remains unchanged. At 7, metres 25, feet the atmospheric pressure is millimetres of mercury and the partial pressure of oxygen is about 59 millimetres of mercury.

Oxygen continues to constitute only The rarefaction of the air at high altitudes not only limits the availability of oxygen for the air breather, it also limits its availability for aquatic forms, since the amount of dissolved gas in water decreases in parallel with the decline in atmospheric pressure. The variations in the characteristics of air and water suggest the many problems with which the respiratory systems of animals must cope in procuring enough oxygen to sustain life.

Respiratory structures are tailored to the need for oxygen. Minute life-forms, such as protozoans, exchange oxygen and carbon dioxide across their entire surfaces. Multicellular organisms, in which diffusion distances are longer, generally resort to other strategies. Aquatic worms, for example, lengthen and flatten their bodies to refresh the external medium at their surfaces.

Sessile sponges rely on the ebb and flow of ambient water. By contrast, the jellyfish, which can be quite large, has a low oxygen need because its content of organic matter is less than 1 percent and its metabolizing cells are located just beneath the surface, so that diffusing distances are small. Organisms too large to satisfy their oxygen needs from the environment by diffusion are equipped with special respiratory structures in the form of gills, lungs, specialized areas of the intestine or pharynx in certain fishes , or tracheae air tubes penetrating the body wall, as in insects.

Respiratory structures typically have an attenuated shape and a semipermeable surface that is large in relation to the volume of the structure. Within them there is usually a circulation of body fluids blood through the lungs, for example. Two sorts of pumping mechanisms are frequently encountered: In air-breathing vertebrates, alternately contracting sets of muscles create the pressure differences needed to expand or deflate the lungs, while the heart pumps blood through the respiratory surfaces within the lungs.

Oxygenated blood returning to the heart is then pumped through the vascular system to the various tissues where the oxygen is consumed. Two common respiratory organs of invertebrates are trachea and gills. Diffusion lungs, as contrasted with ventilation lungs of vertebrates, are confined to small animals, such as pulmonate snails and scorpions. This respiratory organ is a hallmark of insects. It is made up of a system of branching tubes that deliver oxygen to, and remove carbon dioxide from, the tissues, thereby obviating the need for a circulatory system to transport the respiratory gases although the circulatory system does serve other vital functions, such as the delivery of energy-containing molecules derived from food.

The pores to the outside, called spiracles , are typically paired structures, two in the thorax and eight in the abdomen. Periodic opening and closing of the spiracles prevents water loss by evaporation, a serious threat to insects that live in dry environments. Muscular pumping motions of the abdomen, especially in large animals, may promote ventilation of the tracheal system. Although tracheal systems are primarily designed for life in air, in some insects modifications enable the tracheae to serve for gas exchange under water.

Of special interest are the insects that might be termed bubble breathers, which, as in the case of the water beetle Dytiscus , take on a gas supply in the form of an air bubble under their wing surfaces next to the spiracles before they submerge.

Tracheal gas exchange continues after the beetle submerges and anchors beneath the surface. As oxygen is consumed from the bubble, the partial pressure of oxygen within the bubble falls below that in the water; consequently oxygen diffuses from the water into the bubble to replace that consumed. The carbon dioxide produced by the insect diffuses through the tracheal system into the bubble and thence into the water.

The bubble thus behaves like a gill. There is one major limitation to this adaptation: As oxygen is removed from the bubble, the partial pressure of the nitrogen rises, and this gas then diffuses outward into the water.

The consequence of outward nitrogen diffusion is that the bubble shrinks and its oxygen content must be replenished by another trip to the surface. A partial solution to the problem of bubble renewal has been found by small aquatic beetles of the family Elmidae e. Several species of aquatic beetles also augment gas exchange by stirring the surrounding water with their posterior legs. An elegant solution to the problem of bubble exhaustion during submergence has been found by certain beetles that have a high density of cuticular hair over much of the surface of the abdomen and thorax.

The hair pile is so dense that it resists wetting, and an air space forms below it, creating a plastron , or air shell, into which the tracheae open. As respiration proceeds, the outward diffusion of nitrogen and consequent shrinkage of the gas space are prevented by the surface tension —a condition manifested by properties that resemble those of an elastic skin under tension—between the closely packed hairs and the water. Since the plastron hairs tend to resist deformation, the beetles can live at considerable depths without compression of the plastron gas.

One extraordinary strategy used by the hemipteran insects Buenoa and Anisops is an internal oxygen store that enables them to lurk for minutes without resurfacing while awaiting food in relatively predator-free but oxygen-poor mid-water zones. The internal oxygen store is in the form of hemoglobin-filled cells that constitute the first line of oxygen delivery to actively metabolizing cells, sparing the small air mass in the tracheal system while the hemoglobin store is being depleted.

The book lungs contain blood vessels that bring the blood into close contact with the surface exposed to the air and where gas exchange between blood and air occurs. In addition to these structures, there may also be abdominal spiracles and a tracheal system like that of insects. Since spiders are air breathers, they are mostly restricted to terrestrial situations, although some of them regularly hunt aquatic creatures at stream or pond edges and may actually travel about on the surface film as easily as on land.

The water spider or diving bell spider , Argyroneta aquatica —known for its underwater silk web , which resembles a kind of diving bell—is the only species of spider that spends its entire life underwater.

Research has shown that the inflated web serves as a sort of gill, extracting dissolved oxygen from the water when oxygen concentrations inside the web become sufficiently low to draw oxygen in from the water.

As the spider consumes the oxygen, nitrogen concentrations in the inflated web rise, causing it to slowly collapse. Most of the life cycle of the water spider, including courtship and breeding, prey capture and feeding, and the development of eggs and embryos, occurs below the water surface. Many immature insects have special adaptations for an aquatic existence. Thin-walled protrusions of the integument , containing tracheal networks, form a series of gills tracheal gills that bring water into close contact with the closed tracheal tubes.

The nymphs of mayflies and dragonflies have external tracheal gills attached to their abdominal segments, and certain of the gill plates may move in a way that sets up water currents over the exchange surfaces. Dragonfly nymphs possess a series of tracheal gills enclosed within the rectum. Periodic pumping of the rectal chamber serves to renew water flow over the gills.

Removing the gills or plugging the rectum results in lower oxygen consumption. Considerable gas exchange also occurs across the general body surface in immature aquatic insects. The insect tracheal system has inherent limitations. Gases diffuse slowly in long narrow tubes, and effective gas transport can occur only if the tubes do not exceed a certain length.

It is generally thought that this has imposed a size limit upon insects. Gills are evaginations of the body surface.

Some open directly to the environment; others, as in fishes , are enclosed in a cavity. In contrast, lungs represent invaginations of the body surface. Many invertebrates use gills as a major means of gas exchange; a few, such as the pulmonate land snail , use lungs.

Almost any thin-walled extension of the body surface that comes in contact with the environmental medium and across which gas exchange occurs can be viewed as a gill.

Gills usually have a large surface area in relation to their mass; pumping devices are often employed to renew the external medium. Although gills are generally used for water breathing and lungs for air breathing, this association is not invariable, as exemplified by the water lungs of sea cucumbers.

The marine polychaete worms use not only the general body surface for gas exchange but also a variety of gill-like structures: The tufts, used to create both feeding and respiratory currents, offer a large surface area for gas exchange.

In echinoderms starfish, sea urchins, brittle stars , most of the respiratory exchange occurs across tube feet a series of suction-cup extensions used for locomotion. The gills of mollusks have a relatively elaborate blood supply, although respiration also occurs across the mantle, or general epidermis. Clams possess gills across which water circulates, impelled by the movements of millions of microscopic whips called cilia.

In the few forms studied, the extraction of oxygen from the water has been found to be low, on the order of 2 to 10 percent. The currents produced by cilial movement, which constitute ventilation , are also utilized for bringing in and extracting food. At low tide or during a dry period, clams and mussels close their shells and thus prevent dehydration.

Metabolism then shifts from oxygen-consuming aerobic pathways to oxygen-free anaerobic pathways, which causes acid products to accumulate; when normal conditions are restored, the animals increase their ventilation and oxygen extraction in order to rid themselves of the acid products.

In snails , the feeding mechanism is independent of the respiratory surface. Cephalopod mollusks, such as squid and octopus , actively ventilate a protected chamber lined with feathery gills that contain small blood vessels capillaries ; their gills are quite effective, extracting 60 to 80 percent of the oxygen passing through the chamber.

In oxygen-poor water, the octopus may increase its ventilation fold, indicating a more active control of respiration than appears to be present in other classes of mollusks.

Many crustaceans crabs, shrimps, crayfish are very dependent on their gills. As a rule, the gill area is greater in fast-moving crabs Portunids than in sluggish bottom dwellers; decreases progressively from wholly aquatic, to intertidal, to land species; and is greater in young crabs than in older crabs. Often the gills are enclosed in protective chambers, and ventilation is provided by specialized appendages that create the respiratory current. As in cephalopod mollusks, oxygen utilization is relatively high—up to 70 percent of the oxygen is extracted from the water passing over the gills in the European crayfish Astacus.

A decrease in the partial pressure of oxygen in the water elicits a marked increase in ventilation the volume of water passing over the gills ; at the same time, the rate of oxygen utilization declines somewhat. Although more oxygen is extracted per unit of time, the increased ventilation increases the oxygen cost of breathing.

The increased oxygen cost, together with the decrease in extraction per unit of volume, probably limits aquatic forms of crustaceans to levels of oxidative metabolism lower than those found in many air-breathing forms. This is largely due to the lower relative content of oxygen in water and the higher oxidative cost of ventilating a dense and viscous medium compared with air.

Unique Features