.384 THE MECHANICAL FUNCTIONS. 



expanded pectoral fins. It is only the larger fish 

 of this form, such as Rays, which have very volu- 

 minous and powerful pectoral fins for striking the 

 water downwards with considerable force, that can 

 rise with facility without the assistance of an air- 

 bladder. In these, the lateral fins, which are 

 enormous expansions of the pectoral fins, may be 

 compared to wings, their vertical action on the 

 water being similar in effect to the corresponding 

 movements of a bird when it rises vertically in the 

 air. Those fishes which swim rapidly, and fre- 

 quently ascend and descend in the water, are in 

 general provided with the largest air-bladders. 



In studying the varieties presented by the forms 

 of the fins in different tribes of fishes, we find the 

 same constant relation preserved with the particular 

 situations and circumstances in which they are 

 placed. The dorsal fins, which are more especially 

 usefvd for steadying the body, are longest in those 

 fishes which inhabit the most stormy seas. The 

 most voracious tribes, which incessantly pursue 

 their prey, are furnished with most powerful mus- 

 cles, and possess the greatest means of rapid 

 progression. On the other hand, many of the more 

 pacific, and weaker species are studiously guarded 

 by a dense and hard integument, serving as a 

 shield against the attacks of enemies, and often 

 armed with sharp points which are sufficient to 

 repel the most daring assailant. The Balistes is 

 covered with scales of singular hardness, closely 

 set together, and frequently having rough edges. 

 The Ostracion, or Trunk-fish, instead of these 

 scales, is provided with a kind of coat of mail, 

 composed of osseous plates, curiously joined toge- 



