FISHES. 323 



internal gas-bag inflated with a preponderance of nitrogen, 



its function being to assist fishes in rising or sinking by 



alteration of their specific gravity. Sharks and 

 Air-bladder. , . „ , » , ^ , 



chimaeras, as well as a number or bony nsnes, 



are without air-bladders, and it is interesting to note that 

 these include both slow ground -loving forms and rapid 

 surface - swimmers. 



We must now consider certain of the more important 

 functions of fishes. 



Our British fishes breathe, without exception, the dis- 

 solved air by means of gills, in passing over which the 

 . . blood is aerated. These gills are, in adult 

 fishes at any rate, covered with flaps of flesh 

 or gristle, though embryonic sharks show bunches of ex- 

 ternal gills. In some fish, like herrings and some sharks 

 of great size, we find appendages called gill-rakers, the 

 function of which is, like the baleen of whales, to filter 

 the water and retain the minute floating organisms on 

 which these forms feed. In the sharks, moreover, the 

 gills open as so many external slits, usually five in num- 

 ber, but in one British form six. 



Like the higher vertebrates, fish may be classified, with 

 considerable reservation, according to their chief food, 

 as carnivorous, herbivorous, or insectivorous, 

 though the line of demarcation is perhaps 

 even less exact. They vary, as every angler knows, in 

 their degree of fastidiousness — some, as the conger, habitu- 

 ally rejecting all but the freshest of food; while others, 

 like the bass, show a constant preference for stale offal. 

 The crabbers find this distinction between crabs and lob- 

 sters, the latter not objecting to bait a few days old. 

 The sharks are, with the exception of the very large 

 basking forms, exclusively carnivorous, some preferring 

 living prey, others, fewer in number, being content with 

 carrion. Bays subsist, as may be gathered from their 

 flat crushing teeth, largely on crustaceans. It is not neces- 

 sary to enter in detail into the various foods of the dif- 



