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.
The Gastropoda, or crawling mollusks, have a head, and are consequently animals of a higher organization than the Conchifera or bivalve class. Their mantle forms a vaulted chamber over the head and neck, and envelopes the foot or crawling-disk; all these the animal can protrude or draw in at pleasure. The head is of a globular form, with two or four exceedingly sensitive tentacles, arranged in pairs on each side of it, as in the garden snail, which has four, two long and two short. These tentacles, which the snail can push out and draw in at pleasure, are hollow tubes, the walls of which are composed of circular bands of muscle. The tentacles are pushed out by the alternate contractions of these circular bands, but they are drawn in again like the inverted finger of a glove by muscular cords proceeding to the internal extremity of the tentacle from the muscle that withdraws the foot. The structure of the tentacles is the same in all the crawling mollusks; they are most sensitive in the Helix or Snail family, but they are believed to be delicate organs of touch in all.
The Gastropod mollusks never have more than two eyes, either placed on the tips, or at the base of one pair of tentacles; in the snail they may be seen as black points on the tips of the longest pair. In some of the higher Gastropods they are of great beauty, and appear to be perfectly adapted for distinct vision. Organs of hearing were discovered by Dr. Siebold at the base of one of the pairs of tentacles, consisting of vesicles containing a liquid and calcareous otolites, which perform remarkable oscillations due to the action of vibratile cilia. In the Snail and Slug group the number of otolites varies from eighty to one hundred.
The mouth of a Gastropod is a proboscis, with fleshy lips, generally armed with horny plates or spines on the jaws. The Snail has a crescent-shaped cutting plate on its jaw, and a soft bifid lip below, but the tongue is the most remarkable microscopic object in this group of Mollusca. In the terrestrial Gastropods, it is short and entirely contained within the nearly globular head. It is tubular behind, but in front it is spread into a nearly flat narrow plate, traversed by numerous rows of minute recurved teeth, or spines set upon flattened plates; in the Garden Snail or Slug each principal tooth has its own plate. [Fig. 170] represents a magnified portion of a Snail’s tongue by Dr. Carpenter; the rows at the edge are separated to show the structure. The teeth are set close one to another, and are often very numerous. In the Helix pomatia, a snail found in the middle and southern counties of England, they amount to 21,000, and in the great slug (Arion ater), there are 26,800. This kind of tongue only serves for rasping vegetable food. All the Trochidæ, which are marine mollusks that are supposed by some naturalists to live on fuci, are remarkable for the length and beauty of their narrow spiny tongues. [Fig. 171] is a small portion of the tongue or palate of the Trochus zizyphinus, highly magnified; the large teeth of the lateral bands, as well as the small teeth in the centre, have minutely serrated edges. [Fig. 172] shows the Trochus granulatus in the act of crawling.
Fig. 170. The tongue of Helix aspersa.
Fig. 171. Palate of Trochus zizyphinus.