Because they vary the form of the body by alternate contractile and expansive movements.
We may here observe, that the motions in water caused by sea animals of various descriptions, were noticed at an early period by observers, but it is only of late years that they have engaged the particular attention of zoologists.
Because they may support that great expenditure and exertion, which is a necessary consequence of the peculiar abode, and whole economy of these animals. - Blumenba'ch.
Because they are calculated to separate air from water, with which it is always united, and bring it into contact with the blood. It is to be observed, however, that many animals which reside in the water, breathe by means of lungs, and are obliged, at intervals, to come to the surface to respire, such as whales; but there are no animals which reside on the land, and are furnished with gills, which are obliged to return to the water to respire. - Fleming.
Because the air soon dries the fine plumes of the gills, and obstructs the process of respiration and circulation.
Because the former, as in fishes, introduces the air, which the water holds in solution, through the mouth into the gills, and then expels it again through the branchial aperture; consequently, not by inspiring and expiring through the same passage, as in those animals which possess lungs. - Blumenbach.
Because an injury received by the gills of fishes is attended by a considerable effusion of blood; and a fish so killed, will keep much longer in a fresh state, than one on which this operation of bleeding has not been performed.
Because it may protect them from the penetrating influence of the surrounding element.
The pores from which this viscous matter is secreted, arc frequently visible to the eye in fishes: they are connected with vessels which traverse the body under the skin, and contain the fluid. - Fleming.
Because these parts possess neither vessels nor nerves; and, therefore, the whole superficies of the ani-mal'sbody is insensible, and constitutes a dead medium, through which impressions are conveyed to the subjacent living parts. - Blumenbach.
Because scales are not changed, but are perennial; and are said to receive yearly an additional layer, from the number of which the age of the animal may consequently be determined. The scales of sea-water fish are bare, but those on coasts or in fresh water are covered with a mucous or slimy membrane.
In examining these different appendices of the skin, we perceive that they pass, by insensible degrees, into one another, as hair into spines, horns into nails, scales into shells, and crusts into membranes. They have all one common origin, namely,the skin; and,independent of secondary purposes, they all serve for protection. - Fleming.