GENERAL ZOOLOGY AND COMPARATIVE ANATOMY 21 



of the lower forms of Vertebrates, as the Am- 

 phioxus, have no bony spinal column, but only 

 a cartilaginous structure; but the spinal cord is 

 present, looking like the notochord of the embryos 

 of all Vertebrates in the first stages of existence. 



The alimentary canal has its beginning at the 

 oral opening, the mouth, which is armed by the 

 teeth of a great variety of forms in the Fishes, 

 Eeptiles, and Mammals for the securing and re- 

 duction of food preparatory to digestion. There 

 is a digestive stomach and an intestinal canal, 

 which is more or less complicated in the different 

 classes, that leads to the anal opening at the 

 posterior extremity of the organism. 



The circulation is complete in all forms of 

 Vertebrates. The blood is corpusculated, and is 

 generally red in color. The heart has from two 

 to four chambers. 



Respiration is performed by gills in the Fishes, 

 by gills and lungs in Eeptiles, and by lungs only 

 in the Birds and Mammals. 



The external covering in this sub-kingdom pre- 

 sents a great variety of forms. In the Fishes and 

 Reptiles the skin is bare or is protected by scales 

 or spines of great variety in size and shape. In 

 Mammals there is a tough leathery skin or dermal 

 plates, or fur, hair, or bristles. The birds are 

 covered with feathers, which on the wing and 

 tail are enlarged and modified to assist in the 



