Because the organs which are employed for both purposes, resemble the oars of a boat in their mode of action; and, in general, possess a considerable extent of surface and freedom of motion. The former condition enables them to strike the surrounding fluid with an oar of sufficient breadth, to give progressive motion to the body ; and the latter permits the same organ to he brought back to its former position for giving a second stroke, but in a different direction, and without offering so great a resistance. The centre of gravity is so placed, that the body, when in action, shall rest on the oars or swimmers, or be brought by certain means to be of the same specific gravity with the water.
Because they have fins, which balance and keep them level; and tails, which act against the water, and direct them like rudders.
The form of the body in fishes, is infinitely more varied than in reptiles or serpents. In most, however, the body has a vertical direction, i. e. flattened at both sides ; in some, on the contrary, as the rays, it is horizontal, and extended laterally; in others, as the eel, etc., it is more rounded; in some prismatic, or quadrangular. In all, the head and trunk are connected immediately, without being separated by a neck. - Blumenbach.
Because her limited soil, contrasted with extensive sea-coasts, and numerous rivers and lakes, intimate to her population, the expediency of obtaining a large portion of their sustenance from the waters. These are known to teem with life, and to furnish a supply of agreeable and nourishing food, which may be pronounced inexhaustible.
Savage nations, as the Kamschatkadales, Brazilians, etc. possess the art of preparing fish in a great variety of ways, even as a kind of flour, bread, etc. With many, as the Islanders of the Pacific Ocean, fishing forms a principal occupation, and a serious kind of study with reference to the ingenious methods and instruments which they have invented. To a great part of the cultivated world, the taking of the herring, cod, tunny, etc. is of still greater value. The oil of the shark, cod, and herring, is used for burning in lamps, etc. The inhabitants of the eastern coast of the middle of Asia, clothe themselves with the tanned skin of the salmon. Many parts of other fish are employed for the purposes of art, as the scales of the bleak, for making false pearls. Shagreen is made from the skins of sharks and rays.
Because asphyxia (or suspension of pulsation) is occasioned by the sinking of the branchiae, or gills, no longer supported by the interposition of water between their laminae (or layers); and this idea has been confirmed in prolonging the life of fishes, by artificially keeping the laminae in the state of separation which the water produces. On the other hand, by compressing the branchiae under water, similarly to their condition in the air, death occurred as quickly as in the latter fluid.
Water may act on the respiration of fishes chemically, physically, or mechanically. The latter influence has, however, been but imperfectly attended to. In 1830, M. Fleurens, with the view that water exercises only a mechanical action on their respiration, put several fish into wine. They did not live as in water, but their death was much longer delayed than in air. He explained this action of the wine, by remarking that this liquid contains much less air than water.