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grand organ of motion, and that the muscles which form by far the greater part 
of the bulk of the animal, are so inserted upon the processes of the vertebral 
column as to give to this organ of swimming the greatest energy which it can 
possess consistently with the bulk of the animal. But the tail of a fish has no 
motion except lateral motion, that is striking right and left in the direction of a 
plane, which, in the majority of fishes, passes equally through the centre of the 
back and the centre of the belly; and this, though it gives motion, and in many 
instances very rapid motion, has no power of ascent or descent in it, because it 
can strike the water neither downwards nor upwards; and it is by striking the 
water in one direction, that the body of a fish, or any substance immersed in the 
water, is impelled in the opposite direction. The direction of the course of fishes 
thus depends chiefly upon the action of those fins on the under part of the body, 
which answer to the four extremities in the mammalia; and those which have 
only two such fins—and for that reason are called apodal, or footless.—have 
lengthened bodies, and partly direct their motions—which are, generally speaking, 
much slower than those of other fishes—by the contrary flexures of the length¬ 
ened body, as may be observed in the Eels, which have no distinct and separate 
caudal fin, but have the dorsal and the anal continued over a great portion of 
their length, and meeting each other at the extremity, as one continuous fin. 
In the majority of fishes, however, there are four fins on the under part, and 
the different place of the posterior pair, determines, in a great measure, the mode 
of swimming in the animal, and the depth at which it inhabits the water. There 
is, also, a form of the body correspondent to this position of the fins, and to the 
depth of water at which the fish in general inhabits. If it is a surface fish, the 
body is, generally speaking, compressed in its lateral diameter, and the head is 
rather small, so that the centre of gravity falls nearly in the middle of the length, 
or rather midway between the anterior and the posterior fins on the under part, or, 
as they are called, the pectoral and ventral fins. A fish formed in this manner, is 
adapted more for straight forward motion than for rapid ascent and descent; and 
such fishes are furnished with dorsal fins, which, as well as the anal fins, are 
generally produced in proportion as the body of the fish is short and compressed 
according to its depth. Of this form we have examples in the Lancet fishes, and 
a number of others, many of which are vegetable feeders, living upon weeds ; 
and others, again, feed upon the small animals which inhabit, in great numbers, 
the floating sea-weed which remains in the great eddies in the tropical seas. 
Such fishes, as they are not predatory upon any other fishes, are very often armed 
with powerful defences against the attacks of these. Those armatures consist of 
hard and sharp spines situated on various parts of the body-—near the tail in the 
Lancet fishes, on the gill-lids in others, and in the dorsal fin, or in advance of 
it, in those fishes which inhabit the bottom of the waters, as in the Weever. 
In short, those defensive weapons are always so placed upon the body of the fish 
