The clumsy amphibians (amphi, both; bios, life), like Dr. Jekyl and Mr. Hyde, typically lead a double life, that is, first in the water and then on the land. As a class they bridge one of the greatest gaps in vertebrate evolution. The result of this ambitious attempt is that they present a medley of makeshift adaptations, which, while leaving them still a long way from vertebrate perfection, nevertheless make them of particular interest to the student of comparative biology.
Along with access to two different habitats they must encounter a double set oi enemies, but at the same time they have two avenues of escape, land and water.
The earliest record of any walking vertebrate is a single illuminating - footprint, as unique as that which Robinson Crusoe found in the sand of his island beach, left in the Upper Devonian shales of Pennsylvania, and now to be seen in the Peabody Museum of Yale University. The three-toed fossil ancestor of the amphibians that made this famous footprint has been christened Thinopus (Fig. 27).
Among the dual adjustments that any animal living a part of the time submerged in water and a part of the time upon land must make, are those associated with locomotion and protection against desiccation.
In water an elongated fishlike body, propelled by a muscular tail, has proved to be the most efficient mechanism for locomotion. On land such an arrangement would be out of the question, because in thin air a propeller that could develop power enough to move the body at all would necessitate so great an addition of heavy muscles as to defeat the possibility of aerial or terrestrial locomotion. When the weight of the body is no longer supported by a surrounding medium of water, the two pairs of appendages become modified into legs which act as levers ,to lift the body away from frictional contact with the ground. It is quite possible to equip such levers with adequate muscles without adding excessively to the entire weight to be moved.
Amphibians, like all land animals, are therefore tetrapods (tetra, four; pod, foot). At best, however, they are not particularly successful at locomotion on land.
The legs of salamanders, ridiculously small and inadequate, cannot even lift the body from the ground, for instead of being directed ventrally as supports, they project laterally, and can be used only slightly in poking the wriggling body along over the ground. Even in frogs and toads, where amphibian legs reach their highest development, such locomotor appendages are so inefficiently anchored to a single vertebra of the supporting backbone that these animals cannot bear their weight upon them in the sustained manner necessary for standing or walking, and can progress only by the momentary exertion of hopping or jumping. When not locomoting they never stand but always sit.
The problem of protection against desiccation arises from the fact that the air, usually far from saturated with moisture, takes up water rapidly from any moist surface. The cells of the body, as complex chemical laboratories, must contain considerable water to permit metabolic processes to go on. One phase of this problem is that of breathing.
The essential feature of every breathing device is a delicate wall, or membrane, separating blood from the oxygen-containing medium. In submerged animals, gills, thin-walled structures containing blood and hanging in water in which oxygen is dissolved, fulfil this condition. When exposed for any considerable time to free air air, however, thin-walled gills dry up and collapse, making gaseous exchange no longer possible. In animals breathing air, lungs are developed. These are enclosed sacs in which an enormous expanse of capillary blood vessels, behind very thin moist walls, is exposed to air. The drying up of this kind of wall is prevented because openings from the lungs to the outside through which evaporation can occur are relatively small, and also the air on the way to the lungs may be moistened in the respiratory passages.
Amphibians not only utilize gills and primitive lungs in respiration but they also exchange gases to a very large extent directly through the skin, which, so long as it is kept moist, may remain thin enough to serve as the membrane separating the blood from the surrounding air. Consequently these animals can live only in moist places. On the other hand, higher land animals, in which an efficient pulmonary system is formed, are not restricted, because they develop a thick, relatively dry integument which is resistant to desiccation. Relatively inefficient respiratory organs, together with various other anatomical handicaps, prevent amphibians from maintaining a body temperature independent of that of their surroundings. Since they can never be active when it is cold, they are excluded entirely from frigid regions, while in temperate zones, where winter condemns them to hibernation, they are able to exercise seasonal activity for only a part of the time.
The problem of desiccation is also involved in the breeding habits of amphibians because they have not made the changes required of true land animals. In the case of reptiles, birds, and mammals, the embryo very early becomes enclosed in an amnion (Fig. 34), a liquid-filled sac produced by the embryo itself. As this amnion persists until hatching or birth, the embryo is protected in liquid throughout its development. No amnion is produced by embryos of lower vertebrates including the Amphibia. The latter must, therefore, go back to the water to breed in most cases. A few avoid this requirement by various means (Fig. 28). Some tree frogs lay their eggs in rain-filled holes in trees or pouches formed by folding leaves. Others carry their eggs about in various ways in pouches or pits on the body. A few lay their eggs in very moist places, beneath logs or stones.
Furthermore, the metamorphosis of such an amphibian as a frog or a toad, necessitated by its emergence from water to land, works profound changes both in its structure and in its feeding habits. During its lifetime the toad changes its diet six times. While in the egg it absorbs the yolk stored within; then, upon hatching, it develops a temporary mouth and eats its way out through the jelly of the egg envelopes; next it becomes a free tadpole, swimming about by means of a fishlike tail, and feeding mainly upon vegetation found in the water. With the growing pains of its coming transformation it loses its temporary mouth and along with it the appetite for vegetable food. Tiny legs and arms now sprout out through slits in the skin, and instead of swimming about much the little toad sits quietly in its shirtsleeves and devotes itself introspectively to the task of making its tail substance over into more useful parts of the body. By this time cold weather is approaching and it goes into a long winter retirement during which its only food is a pair of fat bodies, peculiar nutritive storage organs attached near the gonads in the body cavity and provided to meet the intervening demands of hibernation. With the warmth of returning spring the young toad, meanwhile equipped with a new mouth and a marvelous lassoing tongue, emerges into a life of carnivorous activity upon land, catching slugs and insects for a living.
It is not very difficult to recognize amphibians, although by the uncritical they are sometimes confused with reptiles. It was Brogniart who in 1804 separated the Amphibia from the Reptilia as independent classes, because the former have fingers and toes without claws; a scaleless skin; two occipital condyles on which the skull articulates with the first vertebra; hind legs attached to the vertebral column by a single sacral vertebra; and young which breathe by means of gills. Reptiles, on the contrary, have claws; scaly skin; a single occipital condyle; two sacral vertebrae; and young which never resort to gill-breathing. In common with the reptiles, amphibians exhibit certain features not found in fishes, including: (1) modification of paired appendages into legs; (2) modification of swim-bladder region into lungs; (3) two completely separate auricles in the heart; (4) development of a middle ear cavity with a bone to transmit vibrations from external tympanic membranes to internal ear.
Living amphibians of approximately 2000 species may be disposed of in three orders: Gymnophiona, Urodela, and Anura. To these should be added the extinct Stegocephali, or “Labyrinthodonts,” including some 200 species so far discovered.
The stegocephalians, whose ancestry has been traced by some biologists back to the lobe-finned crossopterygians, bear a resemblance to living amphibians, although they disappeared from the earth before any known representatives of modem amphibians made their appearance. This fact, as pointed out by Jaekel, is embarrassing when one seeks to establish them as the undoubted ancestors of the amphibians of today. The gap separating these similar groups of animals may sometime be filled by the discovery of intermediate fossil forms.
The stegocephals flourished in the swampy Carboniferous Period, along with giant rushes, mosses, and tree ferns, before there were any birds, insects, or flowers, and when the warm steamy sluggish atmosphere was probably heavily charged with an abundance of carbon dioxide. They have the distinction of being the earliest four-footed air-breathers on the earth, large awkward creatures with an armor of scales on the head (Fig. 29), and with a brain so small that it could have been easily pulled out through the foramen magnum at the back of the skull. The reason for the name “stegocephali” (stegos, roof; cephalon, head) is that the size of the skull by no means indicates the cranial capacity of these stupid beasts, there being a large attic-like space roofed in above the brain-case itself.
Of modern amphibians the naked, legless Gymnophiona are the least familiar. They include about 50 tropical species from Africa, South America, Ceylon, and India, which burrow in the ground. No fossils are known in this order.
In appearance these animals resemble worms (Figs. 30 and 28c), although possessing a vertebral column of as many as 250 vertebrae, and numerous other characteristics which place them unmistakably with the Amphibia.
As examples of the order may be cited the “blind caecilian,” Caecilia, of West Africa, and Ichthyophis, of Ceylon.
Urodeles are newts, mud puppies, and salamanders. They retain their tadpole-like tails throughout life, and many of them never emerge from existence in water, although some do so, living under stones, rotten logs, and in damp situations generally. Most of them undergo a metamorphosis during which the gills are lost but the tail retained. Some, however, retain their gills and spend their entire lives in the water. Necturus (Fig. 31), the mud puppy of the Mississippi River drainage system, is one of these perennibranchs, so named because of their persistent gills (perenni, lasting through the year; branch, gill). Other urodeles are: Amphiuma, the so-called “Congo Snake” of the Southern United States; Cryptobranchus, the “hellbender” of the Ohio River Valley; Amblystoma, the commonest American salamander and one which has been extensively used for research in experimental embryology; and Triton (Fig. 32), which includes both European and American species.
The “black salamander,” Salamandra atra, of Switzerland, is particularly adapted to life in the cold tumultuous waters of the high glacial streams where it lives. Its eggs, only two of which develop at one time, are protected and prevented from being washed away by remaining within the oviduct of the mother, where they hatch and pass through their entire tadpole-hood, reaching a size large enough to insure their safety as independent animals before they are born into the world.
Anurans are frogs, toads, and hylas, that lose their tails before becoming adults. They are the first truly vocal vertebrates. Other amphibians as well as fishes, with the exception of those which make sounds by means of their air-bladders, are silent. These quaint and cheerful singers, moreover, are the first animals with a lacrimal gland, and so are enabled to wink and to shed tears. This does not mean that trouble enters the world for the first time with them, since tears and blinking are primary adaptations for keeping the eyes of land animals clean, rather than serving as machinery for the expression of the emotions.
The most populous genus of toads in North America is Bufo. There are many genera in other parts of the world, particularly in South America which has a greater variety of amphibians than any other continent.
Some years ago the Department of Agriculture in Washington, in a pamphlet on the economic value of the common toad, Bufo americanus, estimated that a single individual in a garden was worth $19.44 as an insect destroyer. With the changed value of the dollar and the added cost of living, this precise governmental figure should no doubt now be increased. Scaphiopus is the American spadefoot toad. Bombinator of Europe is a famous scarlet-bellied toad that escapes attacks of storks because its warning color is associated with a bad taste, as storks have discovered.
The commonest genus of frogs is Rana, several species of which are found in Europe and North America. Xenopus of Africa, and Pip a, of South America, are anurans of particular anatomical interest, as will appear later.
The little tree frogs have adhesive discs at the ends of their fingers and toes that enable them when they leave the water to climb trees where they conceal themselves, persistently sending out their ventriloquistic calls. American genera are: the “cricket frog,” Acris; the “swamp tree frog,” Pseudacris; and the common tree frog, Hyla (Fig. 33).
Many amphibians are remarkable for the ways in which they care for their eggs and young. Some examples are illustrated by the sketches in Figure 28.