Читать книгу The Book of Shells. Containing the Classes Mollusca, Conchifera, Cirrhipeda, Annulata, and Crustacea онлайн

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The Teredo navalis, or ship-worm, has, by its destructive powers, ruined the noblest vessels, and rendered useless the timbers, on which many of the constructions in harbours mainly depend for security; on this account great attention has been bestowed on its natural history and habits. The barnacle, which attaches itself to the bottoms of ships, renders the planks so foul, as to interfere materially with the rate of sailing of the vessel itself. These are only a few of the useful and noxious qualities of these inhabitants of the deep.

The shell with which a Molluscous animal is covered, is absolutely necessary to protect its delicate body from injury; this shell is, in general, composed of much the same substances as bone; but the bone of a bird, or quadruped, is formed by the agency of the blood, and the particles of which it is composed are deposited by that fluid, and again taken up and restored to the circulation, a circumstance which does not take place in the substance of a shell. The shell is formed by the deposition of layer upon layer, in the course of the growth of the animal, and the ridges we perceive on many shells, point out their periodical increase.