78 COMPARISONS OF STRUCTURE IN AKIMALS. 



suckers, that the fly walks so easily up a 

 smooth pane of glass; in flies, however, each 

 limb is furnished with two, or even three 

 suckers, nor are these organs of adhesion 

 confined among insects to flies only — they are 

 found in many other tribes or groups of in- 

 sects, differently arranged and modified ac- 

 cording to the particular use they are destined 

 to serve. 



Still confining our attention to the vertebrate 

 classes, we may now consider what structural 

 modifications of the limbs are presented to 

 us by fishes. "We know that these creatures 

 are formed for an aquatic life, and that they 

 are furnished with fins on the body, and a 

 tail-fin. "The total structure of the fish," 

 says Cuvier, " is as evidently ordered for 

 swimming, as that of the bird for flight ; but 

 the former, suspended in a fluid almost as 

 heavy as itself, has no occasion for large wings 

 in order to support itself. A great number of 

 species have, immediately beneath the spine, an 

 air-bladder, by the compression or dilatation 

 of which, they can vary their specific gravity, 

 and thus gain the power of ascending or 

 descending. They proceed by the action of 

 the tail, which strikes the Water alternately 



