THE GROUPS OF ANIMALS 



277 



essed skin Avith these scales forms the leatherhke natural shagreen of 

 certain costume accessories. 



Above these are five major groups, the true fishes, amphibia, reptiles, 

 birds, and mammals. Differences among these are found in the hard 

 parts of the skin, the form of the limbs, the structure of the heart, and 

 the means of respiration. 



Fig. 252. — Skate. 



Fu 



253. — A fish. {From Carolina Bio- 

 logical Supply Co.) 



The fishes (Fig. 253) are aquatic, and respire by means of gills. The 

 skin usually bears scales, but these are not toothed like the scales of 

 sharks. The skeleton is at least partly of bone. Locomotion is effected 

 by fins (and the bending of the body), and the heart consists of but two 

 chambers (one auricle and one ventricle). 



Fiu. 254. — A saltiniander. 



Fig. 255.— a frog. 



The amphibia are the salamanders (Fig. 254), toads and frogs (Fig. 

 255), and certain legless forms called caecilians. Their skin is smooth, 

 nearly always devoid of scales, thovigh some fossil amphibia were heavily 

 armored. They are nearly all aquatic at least in young stages, and some 

 of them throughout their lives. Division of their habitats between water 



Fig. 256. — A lizard. {From Carolina Bio- 

 logical Supply Co.) 



Fig. 257. — A turtle. {From Carolina Bio- 

 logical Supply Co.) 



and land is what gives the class its name amphiliia. The heart is three- 

 chambered — two auricles, one ventricle. Though the amphibia are of 

 less value to man than are the fishes, frogs' legs are a table delicacy, toads 

 devour many insects, and most orders have contributed material for 

 important biological and medical investigations. 



Reptiles include lizards (Fig. 256), snakes, alligators, turtles (Fig. 

 257), and such fossil forms as dinosaurs. Their skin contains scales or 



