. Animal physiology. Physiology, Comparative; Physiology, Comparative. 492 ACTION OF THE FINS OF FISHES. changing the direction of the motion, is principally due to the side-fins, which represent the arms and legs. The direction of the surface and stroke of these side-fins varies in different species. In the Cod, Halibut, and others, their action ap- pears to be principally directed towards keeping the body in its right position in the water; since, without such an action, the body would be liable to turn over, in consequence of the position of its centre of gravity. In other instances, the pe
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 492 ACTION OF THE FINS OF FISHES. changing the direction of the motion, is principally due to the side-fins, which represent the arms and legs. The direction of the surface and stroke of these side-fins varies in different species. In the Cod, Halibut, and others, their action ap- pears to be principally directed towards keeping the body in its right position in the water; since, without such an action, the body would be liable to turn over, in consequence of the position of its centre of gravity. In other instances, the pectoral and ventral fins move in such a manner as to assist the action of the tail. In the Rays, the pectoral fins are developed to an enormous extent; and being directed horizontally, their action is vertical like that of the wings of a bird. They are furnished with a great number of joints, by which they are rendered very flexible; and their surface may be thus increased during the down-stroke of the fin, and diminished during the up-stvoke. If this were not done, the action of the fins in elevation would exactly counterbalance the effect of their depression; and no movement would be produced. The great power of the pectoral fins of these Fishes seems con- nected with their want of an air-bladder, which causes them to require a constant exercise of force, to keep them up in the water. Their propulsion forwards is chiefly accomplished, as in other Fishes, by the action of the tail. But sometimes the Rays change their position, and swim sideways ; making horizontal strokes with the pectoral fins (whose surface is then vertical), by which they are moved through the water; and sustaining themselves by vertical strokes of the tail, whose surface is then horizontal. 667. The structure of the organs adapted for movement in the air, bears great analogy to that of such expanded fins; and there are instances in which the same instruments may serve both purposes. Thus there are Fishes wh
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