Page:Natural History, Mollusca.djvu/26

 buried, the animal is still enabled to communicate with the water by its respiratory siphon."

Beyond the rudimentary strip of cartilage that in some of the Cephalopoda represents the vanishing spine of the, the have no internal skeleton. But in the great majority of cases, the soft parts are protected and supported by what we may call an external skeleton, of the substance familiar to us as shell. Lime is the essential element of this substance, as it is also of bone: but shell is a carbonate of lime, while the earthy part of bone is a phosphate.

When we consider the beauty and variety that are presented by shells, the important part they play in the economy and habits of the animals, and the use that is made of them in systematic arrangement, it becomes a question of high interest to inquire in what manner they are formed.

"The shells themselves are absolutely deprived of vitality, permeated by no vessels, and as incapable of expansion by any internal power as the rocks to which they are not uncommonly attached; so that the young naturalist is necessarily at a loss to conceive either the mode of their formation, or the origin of all the gaudy tints and external decorations that render them the ornaments of our cabinets.

"The simple apparatus by means of which shells are constructed, is the external membranous layer that invests the body of the mollusk,—the mantle, as it has been termed; and, whatever the form of the shell, it owes its origin entirely to this delicate organ. . . ..

"It is the circumference, or thickened margin of