Page:Natural History, Fishes.djvu/20

6 deep ponds, or tarns, of mountain districts. Of Marine Fishes, some roam the wide ocean, some play around the coral islands of the tropics, others affect the mud, or the sands of the shallows; some linger near the estuaries of great rivers; others come in mighty armies around the coast at particular seasons, retiring again to the deep water in the offing; finally, some habitually keep near the surface, while others rarely rise from the vicinity of the bottom.

The form of a fish is that best calculated to facilitate its progression through a fluid medium. It is commonly that of a spindle, swelling in the middle, and tapering to each extremity. There are, it is true, many modifications of this form; some, as the Skates and Flat-fishes, are flattened horizontally; others vertically, as the Chætodons and the Dory; some are globose, as the Diodons and Sun-fishes; some are drawn out into a serpent-form, as the Eels and Lampreys; and some, as the Ribbon-fishes, resemble in length and thinness the fabrics from which they derive their name. Yet, in all these varieties the normal form may, without difficulty, be traced. The surface of the body is sometimes smooth, or covered with a slimy secretion; occasionally it is armed with bony plates, which are sometimes set with hard tubercles; in a few species the body is covered with spines, which are capable of being laid close to the body, or erected at will; but the general covering of the body forms scales, or rounded plates (apparently horny, but considered by Professor Owen to be more allied to bone), the front margins of which are imbedded in the skin, and the posterior margins are loose and