186 



ANIMALS CLASSIFIED 



gether. But a man has certain parts of fie skeleton, as nails or 

 hair, formed by the skin and in addition po messes inside bones to 

 which the muscles are attached. Some of tl 3 bones are arranged 

 in a flexible column in the dorsal* (the back) side of the body. 

 This vertebral column, as it is called, is distinctive of all vertebrates. 

 Within its bony protection lies the delicate central nervous system, 

 and to this column are attached the big bones*, of the legs and 

 arms. The vertebrate animals deserve morr of our attention than 

 other forms of life because man himself i a /ertebrate. 



The sand shark, an elasmobranch. Note the slits leading from the gills. (From 

 a photograph loaned by the American Museum of Natural History.) 



Five groups or classes of vertebrates exist. Fishes, Amphibians, 

 Reptiles, Birds, and Mammals. Let us see how to distinguish one 

 class from another. 



Fishes. Fishes are familiar animals to most of us. We know 

 that they live in the water, have a backbone, and that they have 

 fins. They breathe by means of gills, delicate organs fitted for 

 taking oxygen out of the water. The heart has two chambers, an 

 auricle and a ventricle. They have a skin in which are glands 



The sturgeon, a ganoid fish. 



