AMPHIBIA 223 



The Skeleton. There are two occipital condyles. The ver- 

 tebrae of the lower forms are like those of fishes, biconcave; 

 those of higher Amphibia are usually concavoconvex. A 

 sternum first appears in this class, as well as a typical vertebrate 

 limb-skeleton. The pelvic girdle is united with the spinal col- 

 umn. There may be four, two, or no limbs, that take the place 

 of fins. They have digits which are generally without claws. 1 

 The tail is temporary in frogs and toads and permanent in 

 other amphibians. Teeth are present in most amphibians, but 

 lacking in the toad. They are small, sharp, and point back- 

 ward. 



Respiration. Amphibians breathe by gills in the larval or 

 tadpole stage, and in the adult forms by persistent gills or by 

 persistent gills and lungs, as in Necturus, or by lungs, as in the 

 salamander. The skin is also an important organ for taking in 

 oxygen and giving off impurities. The frog breathes with the 

 mouth shut, by lowering the tongue and taking in air through 

 the nostrils, then raising the tongue, closing the nostrils, and 

 forcing the air into the lungs. 



Circulation (Fig. 184). Amphibians are poikilothermal. The 

 heart has one ventricle and two auricles. The arteries carry the 

 blood to all parts of the body. The veins from the lungs return 

 the pure blood to the left auricle, and those from the body return 

 the impure blood to the right auricle. The auricles contract 

 and force both pure and impure blood into the ventricle, which 

 forces it out in such a manner that the venous blood goes to the 

 body and the pure blood to the head. 2 



The Nervous System (Fig. 185). The brain of a frog has 

 advanced above that of the fish in the development of the cere- 

 brum, but the cerebellum, which is very small, being, in fact, 

 but a thin lamella, is inferior to that of the fish. 



The skin on the whole surface of the body is provided with 

 tactile nerve-endings. Special taste organs are located on the 

 tongue and mouth. In adult amphibians the nostrils open 

 into the mouth, and Baskett says, "there is much in the 

 arrangement of the mucous membrane of the frog's nose which 

 implies that it smells." The strong odors of some of their ex- 



1 See " Amphibia and Reptiles," Gadow, p. 146. 

 2 Linville and Kelly's " General Zoology," p. 330. 



