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. off clearly 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 
2 
