(1365). Extensively distributed over the surface of the land, or inhabiting the waters either fresh or salt, there exist a very numerous body of Mollusea, differing widely among themselves in construction and habits, but distinguished by a peculiar locomotive apparatus common to the entire class, by means of which they are able to fix themselves to plane surfaces, and to move from place to place by a slow and gliding motion. The Slug, the Snail, the Limpet, and the Whelk afford familiar examples of their general form and external appearance; but species of different kinds are so common in every situation, that it would be wasting the time of the reader to dwell at any considerable length upon their ordinary configuration and usual mode of progression.

The belly.

The belly.

A foot.

A foot.

(1366). Many families of Gasteropoda, as for example the Nudi-branchiata (fig. 266), are absolutely deprived of any shelly defence, the investment of their bodies being entirely soft and contractile. In others, as the Slug (Limax), a thin calcareous plate is imbedded in the substance of their muscular covering. This little shell is contained in a cavity within the mantle, and is quite loose and unattached to the walls of the cell wherein it is lodged. The mode of its formation and growth is exceedingly simple, and from its very simplicity is well calculated to illustrate the formation of shells of more complex character. The floor of the cavity containing the calcareous plate is vascular, and secretes cretaceous particles mixed up with a viscid animal secretion. The material thus furnished in a semifluid state is applied like a layer of varnish to the lower surface of the shell already formed by the same process; and the added layer, soon hardening, increases the thickness of the original plate, while at the same time, as a necessary consequence of the progressive extension of the secreting membrane, which enlarges with the growth of the Slug, each successive lamina of shell is larger than that which preceded it. Thus the extension of the shell in diameter, as well as its increase in thickness, is easily explained.

In these internal shells, however, there is no colouring matter; so that they are uniformly white, and present the same texture throughout.

(1367). As external shells are generally painted upon their outer surface with colours of different kinds variously disposed, in such the process of growth is somewhat more complicated, and in every essential particular resembles that already described, whereby the shells of the Conchifera are extended in size and thickness.

(1368). We choose, as an illustration of the manner in which the external shells of univalves are manufactured, one of the least complex forms, as being best adapted to elucidate this part of our subject. The Patella, or common Limpet, is covered with a simple conical shell that extends over the whole of the dorsal surface of the mollusk. The testaceous shield that thus protects these animals is generally variegated externally with sundry markings of diverse colours, while within it is lined with a smooth and white nacre.

(1369). On making a perpendicular section of one of these Gastero-pods, the entire mechanism by which such shells are constructed and painted is at once rendered intelligible. The whole of the back of the animal covered by the shell is invested with a membranous mantle, like that of a conchiferous mollusk; but different parts of this mantle are appointed to different offices. The extension of the shell is entirely effected by the margin of the mantle (fig. 260,b), which is thick, vascular, and studded with glands appointed to secrete the colouring material that paints the exterior. This thickened fringe of the mantle is firmly glued to the circumference of the opening of the shelly cone: the earthy matter produced by it is added, layer by layer, to the edge of the shell; and wherever coloured glands are situated, this earthy secretion is coloured with a corresponding pigment: in this manner is the shell gradually enlarged, and every additional stratum of calcareous deposit is thus painted at the moment of its formation.

(1370). The growth of the shell in thickness is a subsequent process. After the formation of the outer layer (g) by the edge of the mantle, the general surface of the pallial membrane (a) adds fresh laminae of pearly matter (f) to the whole interior of the testaceous shield, and it is by the accumulation of such colourless depositions that the thickening of the entire fabric is provided for.

Section of Patella.

Fig. 260. Section of Patella.

Helix pomatia, removed from its shell.

Fig. 261. Helix pomatia, removed from its shell.

(1371). When the manner in which the limpet constructs its habitation is understood, the formation of a turbinated or spiral shell is explained with the utmost facility. On extracting a Snail from its abode, all that portion of its body which was covered by the shell is seen to be invested with a thin mantle (fig. 261, a) precisely analogous to that of the Limpet: from this pallial membrane the nacreous lining of the shell exudes. But around the aperture the mantle swells into a thick glandular collar (6), correspondent in function with the margin of the mantle in Patella, and in like manner provided with glands adapted to furnish colouring matter. From the collar, therefore, those layers are secreted by which the extension of the shell is accomplished; and as the deposit is in this case far more abundant in one direction than in another, the shell, as it expands, assumes more or less completely a spiral shape. Wherever glands for secreting coloured pigment exist, corresponding bands or coloured patches are produced as the layers of growth are formed, and the exterior of the shell is thus painted with the tints peculiar to the species.