46 ZOOLOGY. 



the atlas. This structure furnishes a ready means of distin- 

 guishing the skull of an amphibian from that of a reptile. 



52. The limbs, when present, resemble in their structure 

 those of the higher Vertebrata. In the fore-limb there 

 is a humerus, followed by radius and ulna, carpus, 

 meta-carpus, and phalanges; and in the hind limb a 

 femur, tibia, and fibula, tarsus, meta-tarsus, and 

 phalanges. These parts sometimes consist of bone ; in 

 other cases, of cartilage. 



53. Digestion. The mouth is of large size, and is 

 generally furnished with small conical teeth. There is a 

 liver, divided into two lobes; a gall-bladder, pancreas, 

 and spleen. The intestine, which is short, opens, as in 

 the Reptilia, into a cloaca, which also receives the secre- 

 tions of the kidneys and generative organs. 



54. Respiration. The nature of the respiratory organs 

 forms the most distinctive feature of the Amphibia. In 

 the larval or young state, they all possess gills which en- 

 able them to breath the air contained in water. These 

 gills are external filaments, situated on the sides of the 

 neck. In the frogs, toads, and newts, there are two sets 

 of gills ; one external, which soon disappears, and one 

 lodged in an internal chamber, which remains for a 

 longer period. All the Amphibia possess lungs in the 

 adult state. Generally the gills disappear when the 

 lungs are developed, but sometimes the external gills are 

 retained through life, the animal in this case possessing 

 both lungs and gills. 



The nasal sacs always open posteriorly into the mouth. 



55. Circulation. In the young state, so long as they 

 breathe by gills, the circulation of the Amphibia is similar 

 to that of fishes, the heart containing only two cavities, 

 an auricle and a ventricle. When the lungs are de- 

 veloped, however, the auricle is divided, and the heart 

 then contains three chambers, as in the greater number 

 of reptiles. The venous blood from the body passes to 

 the right auricle, and the arterial blood from the lungs 

 to the left auricle; both these chambers pour their con- 



