326 



SYSTEMATIC ZOOLOGY 



FlG. 328. Alligator mississippiensis ; head of 



the American alligator. (Slight;. : trom 



Ludwig-Leunis' Synopsis der Thietkunde.) 



crocodile lives only in salt-water estuaries, but is found among 

 the East Indies and in Australia, the West Indies, Central 

 America, and in Florida. The crocodiles may be distinguished 

 by the breadth of the snout, which is intermediate between that 

 of the gavials and the alligators. The alligator, Alligator vus- 



sissippicnsis ( or lucius ) ( Fig. 

 328) is found only in America 

 and was once very abundant 

 in our Southern states ; it is 

 fast disappearing, owing to the 

 extent to which it is hunted. 

 It feeds on fishes and the 

 smaller mammals, such as the 

 muskrat; it is said to be par- 

 ticularly fond of dogs. It kills 

 its prey by drowning it, — 

 keeping it below the surface of water, — and does not drown 

 itself because of the structure at the back of the mouth already 

 referred to. The caiman closely resembles the alligator and is 

 found in the Orinoco. The eggs of the alligator, about the size 

 of a hen's egg, are provided with a hard shell ; they are laid in 

 sandy mounds, which the animals sometimes construct for them- 

 selves on the banks of streams. When the young hatch, they 

 at once seek the water. 



CLASS V. AVES 



The Aves (Lat. avis, bird), or birds, are sharply marked off 

 from all the other vertebrates by the fact that the bodv is more 

 or less completely covered with feathers ; the.se structures occur 

 in no other group of animals. Thev arc, further, the first warm- 

 blooded animals that we have considered, for the temperature 

 of the blood is kept pretty constantly at about 38 Centigrade, 

 despite variations in the temperature of the bird's environment. 

 The general shape of the bodv is oval with a movable neck, 

 generally relatively long and surmounted by a light head with a 

 projecting beak. In most birds the body terminates posteriorly 

 in a tail provided with stiff feathers, which serve as a steering 

 apparatus in birds that fly. The anterior appendages have the 

 form of wings, which are folded back against the sides ot the 



