114 THE VITAL FUNCTIONS. 



nished, at the entrance of the mouth, with two 

 horny jaws, having a remarkable resemblance to 

 the bill of a parrot ; excepting that the lower piece 

 is the larger of the two, and covers the upper one, 

 which is the reverse of what takes place in the 

 parrot. These, moved by strong muscles, and con- 

 joined with a large fleshy tongue, constitute a 

 powerful instrument for breaking the shells of the 

 mollusca and Crustacea, which compose the usual 

 prey of these animals. In all Ascidice the proper 

 mouth, by which the food is received for the pur- 

 pose of digestion, is an orifice situated internally at 

 the bottom of the branchial sac. 



Fishes almost always swallow their food entire ; 

 so that their jaws and teeth are employed princi- 

 pally as organs of prehension and detention ; and 

 the upper jaw, as well as the lower one, being 

 moveable upon the cranium, they are capable of 

 opening to a great width. The bony pieces which 

 compose the jaws are more numerous than the 

 corresponding bones in the higher classes of verte- 

 brata ; and they appear, therefore, as if their 

 developement had not proceeded sufficiently far 

 to effect their consolidation into more compact 

 structures.* 



Fishes which live upon other animals of the 

 same class having a soft texture, are furnished with 

 teeth constructed merely for seizing their prey, 

 and perhaps also for slightly dividing it, so as to 

 adapt it to being swallowed. These teeth are of 



* Attempts have been made to trace analogies between the dif- 

 ferent segments of the jaws of fishes and corresponding parts of the 

 mouths of Crustacea and of insects; but the justness of these ana- 

 logies is yet far from being satisfactorily proved. 



