254 BIOLOGY FOR BEGINNERS 



4. Larval forms are vegetarian; adults, carnivorous. 



5. The heart is three chambered, and circulation well developed. 



6. The brain, especially the cerebrum, better developed than in 



fish. 



Among the representatives of this curious group, are several 

 common animals. Frogs, toads, tree-toads,' newts and salamanders 

 are all familiar both by sight and sound. 



The Frog. The frog will be taken as a type not only because 

 common and convenient, but also because of the resemblance of its 

 structure to that of the human being. 



In the work with the frog, it is particularly desirable to compare 

 its structure and development with that of the fish, whenever 

 possible, noting those points in which it is more highly developed 

 and the differences which its land life has made necessary in its 

 structure. 



External Features. The frog's body is short, broad, and angular, 

 evidently not as well adapted for submarine locomotion as the 

 fish, nor has it achieved the graceful form of a highly specialized 

 land animal. The covering is a loose skin, colored to resemble its 

 surroundings, and provided with no scales nor hairs, but supplied 

 beneath with many blood capillaries. It is evident that the skin 

 is not for defense like the scaly armor of the fish but attains some- 

 what the same end by its protective coloration. Its thinness and 

 rich blood supply permit a certain amount of respiration to take 

 place through it. Many amphibians absorb water through the 

 skin instead of by drinking. Some secrete a slimy mucus which 

 assists in locomotion and escape from enemies. The head is broad, 

 flat, and attached directly to the body. The nostrils are located 

 near the anterior and connect directly with the mouth cavity, 

 thus permitting them to be used for respiration. They can be 

 closed by a valve-like flap when under water. 



Head Structures. The Mouth. The mouth is enormous and 

 extends literally from ear to ear. This is a very necessary adapta- 

 tion for food-getting as the insects which constitute its principal 

 diet have to be snapped up in this veritable trap. Another strik- 

 ing adaptation for the same purpose is the arrangement of the 



