[Illustration: Fig. 169. Section of nacreous lining of the shell of Avicula margaritacea (pearl oyster).]
In many shells the internal layer has a nacreous or iridescent lustre, shown by Sir David Brewster to depend upon the striation of its surface, by a series of nearly parallel grooved lines. When Dr. Carpenter had dissolved the calcareous matter from a thin piece of nacreous substance, taken from the shell of the Haliotis splendens, or Ear Shell, there remained an iridescent membrane, which presented to the microscope a series of folds or plaits somewhat regular, and splendidly iridescent, but when the plaits were unfolded and the membrane stretched, the iridescence vanished. So the varied hues of mother-of-pearl are owing to the folds of an organic membrane.
The shells of the Gastropoda, or crawling mollusks, have a structure peculiar to themselves, but by no means so much varied as that of the bivalve class. The Strombus gigas, or Queen Conch, the Cassis, or Helmet Shell, and the beautiful porcellanous Cyprææ or Cowries, are much valued by the artists who cut cameos, on account of the structure of their shells, which consists of three strata, the same in composition, but differing in arrangement, and sometimes in colour. Each stratum of the shell is formed of many thin laminæ, placed side by side, perpendicular to the plane of the stratum, and each lamina consists of a series of prismatic spicules with their long sides in close approximation; the laminæ of the inner and outer strata have their spicules parallel to one another, while the spicules of the intermediate lamina are perpendicular to those on each side. According to Dr. Bowerbank, who discovered this complicated structure, the spicules are microscopic tubes filled with carbonate of lime.
The Spondylus gædaropus has sixty ocelli constructed for accurate vision. One can form no idea of the effect of so many eyes, unless they combine to form one image as our eyes do. The common Pecten, or Scallop, pretty both in form and colour, has a number of minute brilliant eyes arranged along the inner edge of the mantle, like two rows of diamond sparks. Some families of mollusks are destitute of eyes, even of the simplest kind; and it has been observed that those mollusks most liberally provided with eyes are also endowed with the most active and vigorous motions. The bivalves do not appear to have either taste or organs of hearing, but they are exceedingly sensitive to touch. It is singular that animals which have neither head nor brain should have any senses at all. A nerve-collar round the gullet with a trilobed nerve-centre on each side supplies the place of a brain; nerves extend from these; besides there are nerve-centres in various parts of the unsymmetrical bodies of the acephalous mollusks.