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attains its full development in other classes, is adapted to discharge some office 

 quite different from that to which it is destined in its perfect state." But how 

 diffidently d« all natviralists speak on this subject ! 



The Air bladder does not exist in aU fishes, and is absent in those sea-fish 

 accustomed to remain at the bottom, and whose movements are slow. Such is 

 the case in the order of Dennopteri, consisting of the Lancelet, Myxine, 

 Lampern, and Lamprey, and in the Pleuronectidze, a sub-order of the Ana- 

 eanthini, consisting of Plaice, Sole, Flounder, Brill, Turbot, HoUbut, and Dab, 

 commonly called "fiat fish." It is easy to understand why, in the order of 

 Pleuronectidse, this organ is absent, because the habits and form of this class of 

 fish are not such as to require it. Flat fish frequent sandy and muddy shores, 

 swimming closely to the bottom, with their white and under sides frequently 

 resting on the mud. Their form also, width without depth, give them additional 

 buoyancy in the water. The air-bkdder is also absent in the large order of the 

 Plagiostomi, divided into the sub-orders. Squall and Raise, the former consisting 

 of Dog-fish, the Shark family, and the Angel-fish, and the latter of the Torpedo, 

 Skate, Thomback, and a variety of Rays. The Shark family are provided with 

 very strong muscles and numerous fins, which compensate in some measure for 

 the absence of an air-bladder, and though capable of vehement efforts, cannot 

 prolong them. It may be added that since the air-vessel at all times requires 

 considerable space in the abdominal cavity, there would scarcely be room in the 

 internal structure for vi^•iparous reproduction. 



The sub-order of Raise swim sideways, like the flat-fish, and in addition to 

 the advantage of having their weight distributed over a considerable surface, 

 some of the species have their body fringed with a web somewhat resembling a 

 continuous fin, or the wing of a bird, which being muscular, assists mnch in the 

 support of the body. Almost the only fresh water fish in which the air-bladder 

 is not found, is the well-known little BuU-head, or Miller's Thumb, the latter 

 name being derived from its flat head, resembling in shape the miller's well- worn 

 and flat thumb. The habit of this fish is to hide under the stones in the river's 

 bed, beneath which it is able to thrust easily its appropriately-formed head, and 

 though it swims away rapidly when disturbed, seldom rises from the bottom. 



There are exceptions, however, to this rule of nature, for which it is 

 impossible to account. We have two Red Mullets without swim-bladders, and 

 yet they seem to possess all the powers in the water of the Indian and American 

 species, which have them. The two British species of Mackerel both swim near 

 the surface of the water with the same ease and swiftness — one has a swim- 

 bladder, the other has not. Of our two species of Orthragoriscus, which appear 

 to possess the same powers — one has the swim-bladder, the other has not. 

 Such exceptions add much to the interest with which this subject is replete, and 

 when we seem to have hit the truth, we find ourselves wide of the mark. 



It will be interesting here to give a description of the swimming bladders 

 in our common fishes, 



