264 



FISHES. 



of osseous fishes. The skull is composed of a single piece, but 

 shaped in other respects very much like that of an ordinary fish. 

 The maxillary and intermaxillary bones do not exist, or are found 

 only in a rudimentary state, concealed beneath the skin. The 

 lower jaw is constituted of one piece on each side, and the oper- 

 cular apparatus is in general entirely wanting. 



Sometimes the gills are free on their external edge, as in the 

 osseous fishes ; sometimes, on the contrary, they are attached by 

 both edges, and from this circumstance the Chondropterygii are 

 divided into two groups 



1. Those with free branchiae (Sturgeons) ; 



2. Those with fixed branchiae (Sharks, Rays, &c.) 



CHONDROPTERYGII WITH FREE BRANCHLE. 



The Sturgeons (Acipenser) have the general form of osseous fishes ; their 

 body is more or less covered with plates of bone embedded in the skin in longi- 



FIG. 283.- THE STURGEON (Acipenser Sturio). 



tudinal rows. Their mouth is adapted for suction and unprovided with teeth. 

 These fishes are generally large, and endowed with considerable muscular 

 strength. They easily stem the most rapid current, and can strike violent 

 blows with their tail ; but their habits are ordinarily peaceful, and they are 

 formidable only to small unarmed prey. They feed on herrings, mackerel, 

 and sometimes salmon, and also root in the mud for worms and Mollusks. In 

 the spring they ascend certain rivers from the sea, often in numerous shoals, 



