THEIR GENERAL CHARACTER AND STRUCTURE 17 



the body of a fish, and to specify its main divisions. Ichthyo- 

 logists, then, recognise four main parts in a fish's body the 

 Divisions of head, trunk, tail, and fins. The head is usually marked 

 the body. o ff dearly from the trunk by the gill-openings. It 

 is only in primitive or archaic forms of fish like the lamprey, 

 where the single pair of gill-openings is replaced by a number 

 of small apertures, that any doubt can arise as to its limits. The 

 boundary between trunk and tail is more arbitrary ; or, at least, 

 is not easily to be recognised in the external form of certain fish. 

 In fish formed after the standard type, like the perch and salmon, 

 the trunk extends from the gill-opening to the vent, and 

 contains the abdominal cavity, with the stomach, intestines, 

 and generative organs. But in the flounder, as has just been 

 described, as well as in some other fish, the vent is placed far 

 forward, and the abdominal cavity, with its important contents, 

 extends far behind it. In such fishes there is no external 

 feature to denote the boundary between trunk and tail. 

 Colloquially, what is really the caudal fin is spoken of as the 

 tail ; but that is merely an appendage to the tail. The true 

 tail is that part of the vertebral column which extends behind 

 and beyond the abdominal cavity. 



Reverting to the perch as our standard or typical fish, 

 it will be observed how well designed is the body for easy 

 External progression through a denser medium than the 

 design, atmosphere. Length, lateral compression, a sharp 

 prow and a fine-run aft, came to be recognised by man, so 

 soon as his idea of navigation got beyond propulsion on a 

 raft, as the desiderata in a vessel that was to make steady, 

 rectilinear motion through an incompressible fluid. All these 

 are manifest in the perch, although, as has been said above, 

 they are brought to greater perfection in fish of migratory 

 habits, such as the salmon, the mackerel, and the herring, 

 which have to travel exceedingly long distances, and therefore 

 have developed lines of the least possible resistance. Compare 

 the outline and contours of any of these fish with those of 



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