HIGH SCHOOL ZOOLOGY. 



73 



breathinir orsan. A fold of mucous membi'anc wliich serves to 

 increase the internal surface of the intestine and known as the 

 ''spiral valve" occurs in all. Finally the vertebral column 

 evidently turns up at the tii> in such a way as to divide the 

 caudal fin unequally or heterocercally 



The American Ganoids fall naturally into two groups accord- 

 ins to the nature of the skeleton : in the Chondrostei it is 

 cartilaginous, in the Holostei osseous. To the former group 

 belong the two families Polyodontidae and Acipenseridse, of 

 which the former includes the Paddle-fish, P. sjxUula (Fig. 47) 



Fig. 47— .The Paddle Fish Polyodon spatula. 

 (U. S. F. C.) 



of the Mississippi, the latter the ordinary Stui-geon of our 

 Lakes {^Acii^enser rubicundus) (Fig. 48) and the Shovel-nosed 



Fig. 48— The Lake Sturg-eon. Adpenser rubicundus. •^. 

 (U. S. F. C.) 



Sturgeon of the Western and Southern States (Scaphirrhyn- 

 ch.ops). In all of these forms the skull is a cartilaginous box 

 adapted to the shape of the brain and sense-organs, and covered 

 with regular bony plates of the same nature as those further 

 back in the body. Polyodon is remarkaljle for its paddle-shaped 

 snout by means of which it stirs up the mud in the river bottom, 

 on the minute organisms contained in which it feeds. These 

 are sifted out by means of the long and close set gill-rakers 

 which form a very efticient sieve for the muddy water which 

 flows out thi'ough the gill-slit. In many respects Polyodon 



