The integuments of the aquatic animals are many in number. Some are covered with a hide and hair, as the sea-calf and hippopotamus, for instance; others again, with a hide only, as the dolphin; others again, with a shell, as the turtle; others, with a coat as hard as a stone, like the oyster and other shell-fish; others, with a crust, such as the cray-fish; others, with a crust and spines, like the sea-urchin; others, with scales, as fishes in general; others, with a rough skin, as the squatina, the skin of which is used for polishing wood and ivory; others, with a soft skin, like the mur?na; and others with none at all, like the polypus.
 
Of all aquatic animals the sea-calf is killed with the greatest difficulty, unless the head is cut off at once. It makes a noise which sounds like lowing, whence the name of “sea-calf.” The animals are susceptible, however, of training, and with their voice, as well as by gestures, can be taught to salute the public; when called by their name, they answer with a discordant kind of grunt.[132] No animal has a deeper sleep than this; on dry land it creeps along as though on feet, by the aid of what it uses as fins when in the sea. Its skin, even when separated from the body, is said to retain a certain sensitive sympathy with the sea, and at the reflux of the tide, the hair on it always rises upright: in addition to which, it is said that there is in the right fin a certain soporiferous influence, and that, if placed under the head, it induces sleep.
 
There are one hundred and seventy-four species of fishes, exclusive of the crustacea, of which there are thirty kinds.[133]
 
Tunnies are among the most remarkable for their size; we 137 have found one weighing as much as fifteen talents (1200 pounds), the breadth of its tail being five cubits and a palm. In some of the rivers, also, there are fish of no less size, such, for instance, as the silurus of the Nile, the isox of the Rhine, and the attilus of the Po, which, naturally of an inactive nature, sometimes grows so fat as to weigh a thousand pounds, and when taken with a hook, attached to a chain, requires a yoke of oxen to draw it on land. An extremely small fish, which is known as the clupea, attaches itself, with a wonderful tenacity, to a certain vein in the throat of the attilus, and destroys it by its bite. The silurus carries devastation with it wherever it goes, attacks every living creature, and often drags beneath the water horses as they swim. It is also remarkable, that in the river Main of Germany, a fish that bears a very strong resemblance to the sea-pig, requires to be drawn out of the water by a yoke of oxen; and i............