SWIMMING BLADDERS IN FISHES. 263 



the ventral (belonging to the stomach) fins serve to raise 

 and depress the fish : when the fish desires to have a retro- 

 grade motion, a stroke forward with the pectoral fin effectu- 

 ally produces it; if the fish desires to turn either way, a 

 single blow with the tail the opposite way sends it round at 

 once; if the tail strike both Avays, the motion produced by 

 the double lash is progressive, and enables the fish to dart 

 forwards with an astonishing velocity. The result is not only 

 in some cases the most rapid, but in all cases the most 

 gentle, pliant, easy, animal motion with which we are 

 acquainted. In their mechanical use, the anal fin may be 

 reckoned the keel; the ventral fins, the outriggers; the pec- 

 toral fins, the oars [and, we may now add, the caudal fin, 

 the screw-propeller]. And, if there be any similitudes 

 between those parts of a boat and a fish, observe," adds 

 Paley, " it is not the resemblance of imitation, but the like- 

 ness which arises from applying similar mechanical means 

 to the same purpose." 



Another powerful aid to the buoyancy of fishes is the air 

 or swimming-bladder, which is described as a philosophical 

 apparatus in the body of an animal. It is easy to see at the 

 back-bone of the herring and other fishes a shining pearly- 

 looking membrane, almost enveloped by the roe or milt of 

 the fish. This is the air or swimming-bladder; and it is of 

 this, as found in the sturgeon, the carp, the ling, and many 

 other fishes, when dried and prepared by certain processes, 

 that the substance called isinglass is manufactured. 



It is the swimming-bladder that serves the fish for rising 

 or sinking in the waters; but in such fishes as reside at the 

 bottom of the sea or never come to the surface, this bladder 

 is almost always wanting. How truly wonderful is this pro- 

 vision of nature! It would be very worthy of inquiry to 

 know by what method an animal which lives constantly in 

 water is able to supply a repository of air. 



The bodies of fishes are nearly the same specific gravity as 



