FOUND LINKb. 107 



of those fishes,, the observer would never for a moment 

 suppose that they presented any features out of the 

 common. Yet a very slight acquaintance with natural 

 history lore proves the singular nature of their position 

 in the fish-class. Every one knows that fishes breathe 

 by gills; that they are cold-blooded; that their "limbs " 

 are represented by certain of their fins (the " paired J> 

 fins) ; and that their bodies are covered with scales. 

 If we add to these facts, the declaration that fishes 

 possess a heart consisting of only two chambers, we 

 shall have nearly completed our definition of the fish- 

 type ; and we might add, lastly, that the nostrils in 

 fishes are typically closed pockets, and do not, as in 

 higher animals, open backwards into the mouth. 



It is necessary for our present purpose to point out 

 that most fishes possess a singular sac or bag, lying 

 just beneath the spine, and called the swimming- 

 bladder, air-bladder, or sound. From the " sound " 

 of the sturgeon we obtain isinglass ; and in a herring, 

 for example, the " sound " may be seen as a silvery, 

 glistening bag, which is removable along with the 

 other organs of the fish when it is " gutted." This 

 bag contains gas, and its use is that of serving to alter 

 the specific gravity of the fish that is, to render (by 

 compression or expansion of the gas) the body of the 

 fish heavier or lighter than the surrounding water. 

 It thus enables its possessor to readily rise or sink in 

 the medium in which it lives. The air-bladders of all 

 fishes, as Dr. Grunther tells us, at first open into their 

 digestive systems by means of a tube or duct. But 



