266 GENERAL ZOOLOGY 



aquatic conditions. The nostrils and ears can be closed, 

 and there is a valve at the back of the mouth so that food 

 may be captured under water without danger of filling the 

 lungs. The teeth are set in bony sockets, and there are 

 bony plates beneath the scales in the skin. The heart has 

 four chambers (two auricles and two ventricles) thus resem- 

 bling the condition found in birds and mammals. 



The only crocodilians in the United States are the alli- 

 gator, Alligator mississippiensis, which lives in the rivers 

 emptying into the Gulf of Mexico, and the American 

 crocodile, Crocodilus americanus, in southern Florida. The 

 alligator may attain a length of twelve or fourteen feet. 

 During the breeding season the males bellow like bulls 

 and give off a penetrating odor from two musk glands in the 

 lower jaw. Nests are constructed by heaping mounds of 

 rubbish in swampy places and eggs are deposited in them. 

 The American alligator is very shy, but some Indian and 

 African crocodiles attack man. 



GENERAL REMARKS ON REPTILIA 



Without much doubt reptiles arose during evolution from 

 aquatic salamander-like ancestors. This seems probable 

 both from the palseontological records and the embryo- 

 logical changes in living forms. Fossil remains in the 

 stratified rocks show that the first vertebrates on earth 

 were fishes, and that amphibians preceded reptiles. A 

 modern reptile during its embryology is at first fish-like, 

 then salamander-like, and in both stages possesses gill 

 clefts which never function -as breathing organs but are 

 lost before hatching takes place. 



Modern reptiles are for the most part truly 'terrestrial 

 animals, but are handicapped to some extent by the fact 

 that they are cold-blooded. They are on this account con- 

 fined to the warmer parts of the- earth, whereas^the birds 

 and mammals may invade the frigid regions. Reptiles 

 have mastered air-breathing and water conservation, so 



