400 



COLLEGE ZOOLOGY 



logic activities are carried on by the use 

 of nutriment stored in the body; and the 

 temperature of the animal is only slightly 

 above that of the surrounding medium. The 

 body temperature of all cold-blooded verte- 

 brates— cyclostomes, elasmobranchs, fishes, 

 amphibians, and reptiles— varies with the 

 surrounding medium. Frogs cannot be en- 

 tirely frozen, as is often reported, since death 

 ensues if the heart is frozen. In v^^arm coun- 

 tries many amphibians seek a moist place 

 of concealment; they pass the hotter part of 

 the year in a quiet, torpid condition; they 

 aestivate. 



Poisonous amphibians 



The poison glands of the leopard frog 

 have already been mentioned. Certain sala- 

 manders and newts are also provided with 

 poison glands. As a means of defense the 

 poison is very effective, since an animal that 

 has once felt the effects of an encounter 

 with a poisonous amphibian will not soon 

 repeat the experiment. Some of the most 

 poisonous species, for example Salamandra 

 salamandra, are said to be warningly colored. 

 Dogs and cats that catch and bite Bufo 

 marinus often die from the toxic effects of 

 their secretions. 



Fossil amphibians 



From fossils, it has been determined that 

 amphibians first appeared during the Age 

 of Fishes in the Devonian period (p. 617). 

 From that time on they increased so rapidly 

 in numbers that the Late Paleozoic or Car- 

 boniferous period is spoken of as the Age of 

 Amphibians. The Paleozoic amphibians are 

 known as Stegocephali, a term that refers to 

 the covered or mailed head, roofed over by 

 dermal bones. Stegocephali were salamander- 

 like animals that probably lived in fresh 

 water or on land. Some of them are called 

 labyrinthodonts because the dentine of their 

 teeth is much folded. Primitive reptiles 

 (cotylosaurs) and perhaps mammals stem- 



med directly from stegocephalians, and the 

 stegocephalians themselves came from cros- 

 sopterygian ancestors. 



OTHER AMPHIBIA 



Legless amphibians 



The family Caeciliidae includes over 50 

 species of wormlike or snakelike legless Am- 

 phibia (Fig. 266). They inhabit the tropical 

 regions of the Americas, Africa, and Asia. 

 They burrow in moist ground with their 

 strong heads and possess eyes that are small 

 and concealed. A sensory tentacle, which can 

 be protruded from between the eyes and the 

 nose, aids the animal in crawling about. 



Giant salamanders 



The family Cr\ptobranchidae contains 

 two genera of giant salamanders. The Amer- 

 ican hellbender, Cryptobranchus alleganien- 

 sis (Fig. 267), occurs only in the streams of 

 the eastern United States; it reaches a length 

 of from 18 to 27 inches. The giant salaman- 

 der of Japan is the largest living amphibian, 

 reaching a length of over 5 feet. 



AxolotI 



Tlie tiger salamander, Ambystoma ti- 

 grinum, occurs from New York to California 

 and south to central Mexico and reaches a 

 length of from 6 to 10 inches. In some parts 

 of its geographic range, it fails to metamor- 

 phose and reproduces while it is in a larval 

 state. Such a larval form is called an 

 axolotl; it was long considered a separate 

 species because the external gills persisted 

 into the adult. However, if an axolotl is fed 

 beef thyroid, even one or two meals, it de- 

 velops into a land animal; it loses its gills and 

 becomes an air-breathing salamander. This 

 is not now thought to be a case of retarded 

 evolution, but a secondary specialization for 

 arid regions. Nonmetamorphosing forms 



