outer side of each, a double row of projections on the inner side. These are 

 called the gill rakers, and apparently serve to keep particles of food or other 

 matter from entering the gill chambers. In the fish'cating fishes the gill rakers 

 are usually very short, in the plankton eaters they are usually numerous and 

 long. 



Air Bladder 



The air bladder or swim bladder lies in the upper part of the body cavity, 

 usually directly beneath the backbone. In the less specialized teleosts an open 

 duct connects it with the gullet. In the more specialized forms the duct is 

 vestigial or absent, the gases in the air bladder being received from the blood. 

 In either case, the air bladder apparently serves as a reservoir of oxygen which 

 can be withdrawn into the blood and replaced by carbon dioxide or by relative 

 ly inert gases such as nitrogen. The air bladder is probably not to be regarded 

 as an ancestral form of lung but as a parallel development. Sharks and lam' 

 preys show no indication of it. In the gars and river dogfish it is usually two 

 lobed. In the higher fishes it may be constricted to form an anterior and a 

 posterior chamber. A few fishes, especially those like the darters which have 

 become adapted to life in swift streams, have the air bladder much reduced or 

 absent. 



Weberian Apparatus 



The characins, minnows, suckers and catfishes have a peculiar modifica- 

 tion of the first four vertebrae. These are usually partly fused and sections 

 of them form a linked chain of bones connecting the air bladder with the canals 

 of the inner ear. When first discovered, these bones were regarded as equiva' 

 lent to the auditory ossicles of the mammalian middle ear, but that has since 

 been disproved. The function of the Weberian apparatus is still uncertain. 

 The suggestion that it affords the owner information as to depth and pressure 

 of water seems to be discounted by the fact that no marine fishes, which might 

 move in greatly different depths, have this apparatus. 



Skeleton 



In scientific studies of fishes a knowledge of the fish skeleton, as well as 

 the rest of its anatomy, is desirable. The bones of the skull and shoulder 

 girdle differ decidedly among the different families and frequently form the 

 basis for taxonomic arrangement. 



OUTLINE OF CLASSIFICATION OF NATIVE FRESH-WATER 

 FISHES AND LAMPREYS 



Class CYCLOSTOMI (or Cydostomata) Lampreys 



With funnel-shaped buccal cavity; no jaws; no bones; no operculum; no 

 paired fins 



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