Carvilius Pollio, a man of prodigal habits and ingenious in inventing the refinements of luxury, was the first to cut the shell of the tortoise into laminæ, or thin slices, and to veneer beds and cabinets with it. CHAPTER VI. DISTRIBUTION OF AQUATIC ANIMALS INTO VARIOUS SPECIES. 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 muræna; and others with none at all, like the polypus. [Illustration: GROUP OF SEALS.] 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]