FRIENDS AND RELATIVES OF FISHES. 85 



portance in the economy of such species as possess 

 it (and these are by far the greater number), yet 

 this swimming bladder is not indispensable to the 

 class of fishes, of which in truth about a fourth part 

 are naturally destitute of it. In the Pleuronectidw 

 or flat-fish, it is entirely wanting, and these species 

 generally remain at the bottom. The Lamprey is 

 also in a similar predicament, and dwells in the 

 mud. It is however difficult to determine for what 

 reason this organ should have been denied to so 

 many fishes, not only of the more indolent kinds, 

 like the majority of those just referred to, which 

 dwell composedly at the bottom of the water, but 

 to many others which yield to none of their class 

 in the ease and velocity of their movements. Its 

 presence or absence does not even accord with the 

 other conditions of organisation ; for while it is 

 wanting in one kind of mackerel, it occurs in 

 another, of which the habits are analogous, if not 

 the same. 



SECTION XVI. 



The general position and relationships of Fishes, consi- 

 dered as a great class in the Animal Kingdom. 



IT results not less from the preceding general 

 exposition of structure, than from all observation 

 of special organisation, that fishes form a class of 

 creatures distinct from every other, and destined 



