Page:Natural History, Mollusca.djvu/29

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of the pallial membrane; the former we have found to be more or less covered by glands appointed for the purpose, situated in the circumference of the mantle, but as these glands do not exist elsewhere, no colouring matter is ever mixed with the layers that increase the thickness of the shell, so that the latter always remain of a delicate white hue, and form the well-known iridescent material usually distinguished by the name of nacre, or mother of pearl."[1]

This lucid description of the process specifically applies to the Conchifera, or Bivalves; but the formation of the shell in the Gasteropoda is not marked by any important point of difference.

No species of this great Division of animals is furnished with limbs, properly so called: unless we may consider as such the long flexible tentacular arms of the Poulpes and Cuttles (Cephalopoda) which are used as instruments of an ungainly sort of crawling, as well as for seizing prey and dragging it to the mouth: yet various modes of locomotion are by turns practised among the Mollusca. In one extensive Class, the Gasteropoda, of which the Limpet and the Snail are

examples, an even gliding movement is that which is most characteristic; a broad expanded muscular disk, called the foot, being applied to the surface over which the animal crawls. Many of the aquatic members of the Class are able to float at the surface by the aid of the same organ. They crawl to the top of the water up the stem of a plant, or the side of a rock, and stretching out the bottom of the foot along the surface, the back being downward, it presently dries by contact with the

  1. Jones's Animal Kingdom, p, 385.