OUTLINES OF ICHTHYOLOGY. 161 



important in fish structure, and which is yet 

 curiously enough absent entirely in many species. 

 Other families have, however, under the spine, in 

 the abdomen, an air- or " swimming "-bladder, 

 varying in shape in the various species, which they 

 can expand or contract at pleasure. This bladder 

 which is more general amongst fresh than salt- 

 water fish is believed to be for the purpose of 

 enabling them to alter their specific gravity to suit 

 the densities, at different depths, of the water in 

 which they desire to swim or suspend themselves. 



Many species, notwithstanding, though wanting 

 this apparatus, have very nearly the same habits 

 as those which possess it ; and in some of these 

 latter there is no external passage by which the 

 air in the bladder can be inspired or expressed. 



Some bladders are composed of a single cham- 

 ber or cavity ; others of two chambers, and a few 

 of three. 



Sexual Distinctions. If we omit one or two 

 species, such as the Tench, there are usually no 

 very obvious external signs by which the sexes of 

 fish can be identified except when in spawn. 

 The distinguishing peculiarities, however, which, 

 with a little practice, will be found useful as 

 guides, are as follows : In the males, the respi- 

 ratory organs, or gills, are frequently larger than 

 in the females, and, on the other hand, the abdo- 

 men or stomach, is smaller. The males may, 



M 



