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Show 324 MOLLUSCA. CLASS III. GASTEROPODA. The Gasteropoda constitute a very numerous class of the Mollusca an idea of which is afforded by the Slug. They 'generally crawl upon a fleshy disk, situated under the abdomen which sometimes however assumes the shape of a sulcus or ~hat of a vertical lamina. The back is furnished ' . with a mantle which is more or less extended, takes variOus forms and in the greater number of genera, produces a shell. Thei; head placed anteriorly, is more or less visible, as it is the more or' less involved under the mantle; I. ts tentacuI a . are very small, situated above the mouth and do not surround it, varying in number from two to six; sometimes they are want· ing; their function is that of touch, or at most that of smell. The eyes are very small, here adhering to the head,, and there to the base ' side ' or point of the tentaculum; sometimes .th ey ar.e wanting. The position, structure, and nature of their respi-ratory ~rgans vary, and afford the means of dividing them into several families; they never, however, have more than a single aortic heart, that is to say, one placed between the pul-monary vein and the aorta. . The position of the apertures, through which the gemtal organs and that of the anus project, varies; they are almost always, however, on the right side of the body. Several are entirely naked ; others have merely a concealed shell, but most of them are furnished with one that is large enough to receive and shelter them. · The shell is formed in the thickness of the mantle. Some of them are symmetrical and consist of a single piece; others GASTEROPODA. 325 are non-symmetrical, which, in those species where they are very concave, and where they continue to grow for a long time, become obliquely spiral. If we figure to ourselves an oblique cone, in which other cones, always wider in one direction than in the others, are successively placed, it will be easily seen that the convolution of the whole takes place ou the side which enlarges the least. This part, on which the cone is rolled, is termed the columella; it is sometimes solid, and sometimes hollow. When hollow, its aperture is called the umbilicus. The whorls of the shell may either remain in one plane, or incline towards the base of the columella. In this last case the preceding whorls rise above each other, forming the spire, vvhich is so much the more acute, as the whorls descend more rapidly, and the less they increase in width. These shells with a salient spine are said to be turbinated. When, on the contrary, the whorls remain nearly in the same place, and do not envelope each other, the spine is flat, or even concave. These shells are said to be discoidal. When the top of each whorl envelopes the preceding ones, the spire is hidden. The part through which the animal appears to come out is named the aperture. When the whorls remain nearly in the same plane, while the animal crawls, its shell is vertical, the columella crosswise on the hind part of its back, and its head passes under the edge of the opening opposite to the columella. When the spire is salient, it inclines from the right side in almost every species; in a very few only does it project from the left when they are in motion; these are said to be reversed. It is observedv that the head is always on the side opposite to that to which the spire is directed. Thus it is usually on the left, and in the 1·eversed on the right. The case is reversed with respect to the organs of generation. The organs of respiration, which are always situc1ted in the last whorl of the shell, receive the ambient element ft·om under \ |
