Page:The American Cyclopædia (1879) Volume I.djvu/468

436 AMPHIBIA for swimming; feet four; cranium solid; tail compressed; respiration by lungs only; gills absent ; no metamorphosis known. Gen- era, menopoma, amphiuma. Order 5. APODA. Body elongated, slender, and snake-like ; feet none; tail very short, almost wanting; lungs, one larger than the other; ribs very short; breast bone wanting ; impregnation unknown, probably internal. The existence of gills at any period of life is unknown. Genus, ccecilia. In the adult amphibia the skeleton varies considerably. In the frog, which moves ex- tensively on land, we find the hind legs greatly developed to enable it to take the enormous leaps by which it pursues its prey and escapes from danger ; hence it has no useless tail, the body is contracted longitudinally into a short space, and the vertebras are few, united into a single immovable piece unprovided with ribs. On the contrary, the water salamanders or newts have a long tail, a slender flexible body, and all their organs fitted for aquatic life. The structure of the bones is more compact and calcareous, and less transparent and flex- ible, than in fishes. The bones of the skull do not overlap each other, but have their margins in contact and occasionally united ; the bones of the face are more closely united to those of the skull than in the fishes ; the intermaxillary and jaw bones are much developed transverse- ly, expanding the general form of the skull without involving any enlargement of the brain cavity, which is very small. The hyoid bone experiences most remarkable changes in those genera which undergo metamorphosis. In the highest amphibia, the bones of the spine are very few ; in the frog there are 9, in the pipa only 8, unprovided with ribs, but having long transverse processes ; the anterior artic- ular surfaces of the bodies are concave, and the posterior convex. In the tadpole, and in the genera which retain their gills through life (siren, menobranchus, &c., hence called peren- nibranchiate), the substance between the ver- tebrae is soft, and contained in cup-like hollows formed by the concave articular surfaces of contiguous bones, precisely as in fishes; an- other proof of the intermediate condition of these lower forms of the class. In the sala- manders the vertebra are more numerous, especially those of the tail ; in the siren the trunk has 43, and the tail as many more. The limbs are arranged on very different plans in the different groups; in the frogs and toads they are well developed, and sailed for active leaping and swimming ; in the elongated aqua- tic species the limbs are small and feeble ; in ceecilia, there are not even the rudiments of limbs ; in these the imperfect development of the extremities is compensated by the extent of the spine of the body and tail, by the curves and flexures of which they swim with consid- erable rapidity. InHhe fish-like tadpole, the limbs are at first scarcely perceptible, and in their gradual development acquire a consider- able size and well-defined form while yet im- prisoned beneath the skin ; the hind legs are first seen ; the tail is much developed, gradu- ally disappearing by absorption as the limbs acquire their full size. All the amphibia have teeth on the palate ; the salamanders have them also in both the upper and lower jaws, the frogs in the upper only, and the toads in neither ; the jaw teeth are always slender, sharp-pointed, and closely set; the frog has about 40 on each side of the upper jaw ; the salamander has about 60 above and below ; the palatine teeth are generally arranged trans- versely, parallel to the jaw teeth. In the adult amphibia the gullet is wide and short; the stomach is a simple sac, elongated in the aqua- tic species ; the intestine is but slightly con- voluted, and terminates in a cloaca, or pouch, which also receives the openings of the genital and urinary organs. The vent in the frogs and toads opens on the hinder part of the back ; in the other forms it is beneath the commence- ment of the tail, as in ordinary reptiles. The liver, pancreas, and spleen are found in all the class. The lymphatic and lacteal systems are extremely developed ; in the course of the lym- phatics are found pulsating cavities or ventri- cles which propel their contents toward the veins, but their pulsations do not coincide with those of the heart, nor those of one side with those of the other. In the circulating system we find the most convincing proof of the inter- mediate position of the amphibia ; they begin life with the single heart and gills of fishes, but as their metamorphosis goes on, the heart as- sumes the compound character necessary for the pulmonary respiration of the reptiles. The heart in the early stage of these animals con- sists of a single auricle which receives all the blood after its circulation, and a single ventri- cle which sends it through a third cavity, the liilbm arteriosus, to the gills or branchiro ; in the gills the blood is purified by the oxygen of the water, and returned by their veins to the aorta ; from the last branchial artery a branch is given off" on each side, which goes to the ru- dimentary lungs and afterward becomes the pulmonary artery ; between the branchial ar- teries and veins are small communicating branches, which, as the gills are absorbed, gradually enlarge until they form continuous trunks, and the original branchial artery be- comes the root of the two descending aortas, at the base giving off the pulmonary artery ; the two veins which return the blood from the rudimentary lungs enlarge as these organs are developed, and become the pulmonary veins, which last, being distended at their point of union with the heart, ultimately form the second auricle. The circulation, then, of the adult amphibia assumes the character which we find in the reptiles generally, being per- formed by a single ventricle and two auricles ; the pure blood in the pulmonic auricle and the impure blood in the systemic auricle are sent together and mixed in the single ventricle, so that a half-purified blood is distributed to tho