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One of the four classes composing the superclass Tetrapoda of the subphylum Vertebrata, the other classes being Reptilia, Aves, and Mammalia. The living amphibians number approximately 2460 species, and are classified in three orders: the Anura or Salientia (frogs and toads, slightly less than 2000 species); Urodela or Caudata (salamanders, 300 species); and Apoda or Gymnophiona (caecilians, about 160 species). The orders in the subclasses Labyrinthodontia and Lepospondyli existed in the geologic past and are now extinct. A classification scheme for the Amphibia follows:
- Class Amphibia
- Subclass Labyrinthodontia
- Order: Ichthyostegalia
- Subclass Lepospondyli
- Order: Nectridea
- Subclass Lissamphibia
- Order: Anura
A typical amphibian is characterized by a moist, glandular skin, the possession of gills at some point in its life history, four limbs, and an egg lacking the embryonic membrane called the amnion. See Amnion, Anamnia
The closest relatives of the amphibians are the fishes, from which they evolved, and the reptiles, to which they gave rise. Present-day amphibians, however, are highly specialized animals, rather different from the primitive forms that probably first arose from crossopterygian fishes and far removed from those that gave rise to the earliest reptiles.
In general, modern amphibians as adults differ from fishes in lacking scales, breathing by means of the skin and lungs instead of gills, and having limbs in place of fins. There are many exceptions to these generalizations, however. Reptiles usually have a dry, scaly skin that is relatively impervious to water loss and very different from the amphibians with their moist skin that permits much evaporation. Young (larval) amphibians have gills, but there is no comparable gill-breathing, larval stage in the life history of a reptile. A most important difference between the two groups is the absence of the amnion in the Amphibia, and its presence in the Reptilia. Lacking this membrane, amphibian eggs must be laid in water or in very moist places. The amnion of the reptile egg makes it more able to resist desiccation, and the eggs can be laid in relatively dry places. The ability to resist water loss through the skin and the development of a land egg are perhaps the differences between reptiles and amphibians that are of the greatest evolutionary significance. See Reptilia
The all-important factor in amphibian life is water. Most species must return to the water to breed, and all must have access to water (even if only in the form of rain or dew) or die of dehydration in a short time. An important consequence of this basic fact of physiology is that vast arid and semiarid areas of the Earth are inhabited by a relatively few specialized amphibians. The majority of amphibian species are found in moist, tropical regions.
Amphibians are among the so-called cold-blooded animals; that is, the temperature of the body of an amphibian is not regulated internally to a high level as is that of mammals and birds, but fluctuates with that of the environment. An animal such as an amphibian that burns none of its food energy in keeping warm is able to get along on much less food than a bird or mammal of similar size. This advantage is offset by the inability of amphibians to be active under cold conditions that do not inhibit a warm-blooded animal. Thus the far northern and southern parts of the world which support large populations of birds and mammals are almost devoid of amphibian life. The amphibians mark a significant point in the evolution of the vertebrates, the transition from aquatic to terrestrial life. As animals neither divorced from the water nor fully at home on land, they suffer from their intermediate mode of life. Reptiles, and later mammals, came to dominate the land, and fishes the waters, leaving the amphibians of today as a relatively unimportant but nevertheless highly interesting group of vertebrates. See Thermoregulation
The fossil record of the three groups of living amphibians is extensive, and the earliest member of each has been found in Mesozoic rocks. However, no intermediary forms linking the three groups together have been found in the Mesozoic, and it is necessary to look in the Paleozoic, some 100 million years earlier, for the common ancestor of modern amphibians, with the earliest known amphibians having been found in the Upper Devonian rocks of Greenland.
It is clear that modern amphibians have a very long history extending back almost to the time of the origin and radiation of land vertebrates 340 million years ago. Their unique sensory biology and specialized glands must have evolved at that time and remained unchanged to the present day.
a class of vertebrates. Amphibians were the first vertebrates to transfer from aquatic to aquatic-terrestrial life. They deposit eggs similar to fish, since their eggs and embryos lack adaptations for terrestrial development (Anamniota). Development ends with metamorphosis, during which the larvae lose their resemblance to fish and are transformed into adult animals. The organization of amphibians as terrestrial vertebrates is in many respects imperfect: their metabolic rate is very low and the body temperature is variable, corresponding to the temperature of the external environment.
Existing amphibians comprise 2,850 species in three orders: Gymnophiona (limbless amphibians), Caudata (tailed amphibians), and Salientia (tailless amphibians). The Gymnophiona have an elongated body; the extremities and tail are absent. The Caudata have an elongated body, a well-developed tail, and usually weak, short limbs. The Salientia have two pairs of limbs and move on the ground by leaps. In the water they swim, propelling themselves with their hind limbs, which in most species are equipped with a web.
The soft, moist skin of amphibians plays an important role in respiration. The moisture of the skin, which is necessary for gas exchange, is maintained by secretions of the mucous glands. On the dorsal side of the body there are large serous glands whose secretions are poisonous. Only some Gymnophiona have tiny bony scales in their skin. There are two occipital condyles. The hyomandibular is transformed into a stapes. During metamorphosis the branchial arches are reduced and transformed, along with the inferior elements of the hyoid arch, into the hyoid bone. The total number of vertebrae varies from nine (in most Salientia) to more than 200 (in Gymnophiona). In the majority of Salientia the ribs are completely reduced. The limbs are paired and of the pentadactyl type. The ilium of the pelvis (greatly elongated in the Salientia) articulates with the transverse processes of the sacral vertebra.
The amphibian brain has a well-developed forebrain, whose hemispheres are completely separated. The cerebellum is weakly developed. There are ten pairs of cranial nerves. The larvae have lateral-line organs. Eye accommodation is accomplished by shifting the lens. In the Gymnophiona that live in soil and in cave Caudata the eyes are underdeveloped. The Salientia have, in addition to an inner ear, a middle-ear cavity and a tympanic membrane. The tactile organs are well-developed. The olfactory organs are well-expressed in the Gymnophiona and Caudata. The organs of taste are weakly developed.
All adult amphibians feed exclusively on animal food. The teeth serve only to grasp and hold the prey. Teeth are entirely absent in the Bufonidae and Pipidae. In contrast to fish, amphibians have a mobile tongue, rich in glands that secrete a sticky mucus that aids in ensnaring small prey. The amphibian digestive tract is relatively short. Most adult amphibians have lungs, internal nares, and laryngeal cartilages. Owing to the absence of a thorax, the air in most amphibians is sucked into the buccal cavity through the nares when the floor of the cavity is lowered; then the nares are closed by valves and the floor of the buccal cavity is raised to the palate, pressing the air into the lungs through the glottis. Pulmonary respiration is supplemented by cutaneous respiration, which sometimes has predominant significance (for example, in lungless salamanders). The Caudata that live in water breathe through gills, retaining them throughout the course of their lives.
The blood circulation of amphibian larvae resembles that offish. The amphibian heart after metamorphosis acquires a three-chambered structure, that is, it is formed of two auricles and one ventricle. The right auricle receives venous blood and the left, arterial blood (from the lungs and skin). The venous and arterial bloods mix only partially in the cavity of the ventricle, whose walls have a complex system of muscular trabeculae. The pulmonary veins receive mainly venous blood, the aortic arches are filled with mixed blood, and only the carotids receive arterial blood.
Adult amphibians have a paired mesonephros. In larvae the so-called pronephros functions in the early stages of development. The urinary bladder is extremely important in water exchange. Males have paired testes and females paired ovaries. Ova are transported through oviducts that connect with the cloaca. Amphibians attain sexual maturity most often by their third or fourth year. The majority deposit eggs in bodies of water. During the breeding season, “spring concerts” (the calls of the males) are characteristic of the Salientia, while mating games are characteristic of the Caudata. Many forms exhibit intensified sexual dimorphism. In almost all Salientia and in a few Caudata fertilization is external; in most Caudata and in Gymnophiona it is internal. Only a few amphibians are viviparous; the rest deposit eggs. The fertility of amphibians varies from three eggs to 28,000. The Salientian larva (called a tadpole), in particular, greatly differs from the adult individual. During metamorphosis, the external gills disappear and the sense organs are restructured to correspond to the conditions of an air environment; the stratum corneum develops in the epidermis. Sometimes development without metamorphosis, that is, direct development, is observed in amphibians that deposit eggs on dry land. The larvae of some Caudata may attain sexual maturity before metamorphosis (neoteny). The majority of amphibians, upon depositing their eggs, abandon them. In some forms the parents transport the young or the eggs on their bodies (for example, the male midwife toad). Some amphibians carry their young until they develop fully (for example, the male Darwin’s frog and the females of the genus Nototroma and family Pipidae).
Most amphibians are beneficial, since they destroy agricultural pests and serve as food for other animals. In many countries (France, Italy, USA) some frogs are used by man as food. Some amphibians, for example, frogs, are the classical objects of physiological experiments.
Fossil amphibians are rather more numerous and diverse than existing ones. The classification of fossil amphibians is based on the structure of their spines and skulls. The most ancient and primitive amphibians—Ichthyostegalia—bore a considerable resemblance to the Crossopterygii (fish), being their descendants. The Labyrinthodontia constituted the main branch of fossil amphibians; apparently, the Batrachosauria, Salientia, and Lepospondyli as well as the Microsauria branched off from them. The Batrachosauria are the probable ancestors of reptiles. Appearing in the Devonian, amphibians attained wide distribution and diversity in the Carboniferous period owing to the hot, humid climate of that period; many terrestrial forms appeared in the drier Permian. In the Triassic the diversity of amphibians, represented predominantly by aquatic forms, was sharply curtailed. The second stage of their relative flourishing belongs to the Cenozoic. Some fossil amphibians attained gigantic dimensions, with a skull measuring more than 1 meter long (Mastodonosaurus). The principal sites of remains of ancient amphibians (Devonian-Triassic) are known in the northern hemisphere, with a few isolated finds in South Africa and India. In the USSR the remains of fossil amphibians (Eogyrinus, Eryops, Gerrothorax, Seymouria, Metoposaurus, Ophiderpeton, Diplocaulus, Cardiocephalus) are numerous in the eastern portion of the Russian platform and serve as reliable guiding forms in establishing the geological age of deposits.
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