The marine mollusca include genera of all the classes. Some, as many of the Cirrhipoda, instead of living in the water, prefer a situation where they may only be bathed occasionally with the flood tide. Others burrow in the sand, or adhere to the rocks which are left dry by the receding tide. These are termed littoral species. Many, however, which have been denominated pelagic, reside in the deep, and are seldom obtained but by dredging, or when thrown ashore during storms.
The effect of temperature in regulating the distribu tion of molluscous animals has not been investigated with any degree of care or success. Over the terrestrial and fluviatile species it probably exercises a very power ful control, greatly limiting their geographical range. In proof of this, it may be stated that the south of France possesses several species not to be found in England, while in England there are a few which have not been detected in Scotland. But among the marine mollusca, the influence of climate is not felt in the same degree. Living in an element whose bulk and motions guard it equally from the extremes of heal or cold, these animals, like the sea-weeds, have a very extensive latitudinal and longitudinal range. Thus, some are common to Greenland and the Mediterranean, others to Britain and the West Indies. The mollusca of the tropical seas, however, differ widely as a whole from those of the temperate re gions. Some of the forms appear to be peculiar to warm regions, and in general the intensity of colour decreases as we approach the poles. But as there have been few cultivators of this branch of science, the geographical distribution of the species has been but imperfectly ex plored. How few parts of either England or Scotland have been surveyed by the eye of the helmonthologist, so that many species whose range is considered as limit ed, may ere long be found to be extensive.
If the observations are few and imperfect, which have been made on the influence of temperature in re gulating the physical distribution of mollusca, we are still in greater ignorance with regard to the power of habit. In the flcetz rocks, the relics of marine and fluviatile mollusca are found mixed in the same bed. This circumstance gave rise to the inquiry, how far the mollusca of Fresh water can be habituated to sea water, and vice versa. In the account of the proceedings of
the National Institute of France for the year 1816, we are informed that M. Beuchant, Professor at Marseilles, has directed his attention to this subject. He found that all these animals die immediately, if we suddenly change their place of abode ; but that, if we gradually increase the proportion of salt in the water for the one set, and diminish it for the other set, we can, in general, accustom them to live in a water which is not natural to them. He found, however, some species which resisted these attempts, and which could not bear any alteration in the quality of the water in which they reside. Before much confidence can be placed in the accuracy of these results, it would be desirable that the experiments were repeated by other observers. There arc, indeed, many sources of error to be guarded against. When we change animals from fresh to salt water, or from salt water to fresh, we must necessarily derange their mo tions, by compelling them to reside in a medium of a dif ferent degree of density from the one which they have been accustomed to dwell in, and to which the arrange ment of the different parts of the body is adapted. By such a change of place, it would be difficult for those which breathe air to come to the surface, and descend again in their new situation. In those with gills, the ap plication of a new kind of fluid to the surface of such delicate organs would considerably influence the func tion of respiration. The change of situation would like wise be accompanied by a corresponding change of food, and consequently, not merely the organs of loco motion and respiration, but likewise those of digestion, would suffer a derangement in their operations. We know that the power of suffering in the animals of this class is very gicat,,and that they survive, though sadly mutilated. Some of the snails will live in a quiescent state for years, without food, and almost without air. Un less, therefore, the animals subjected to these experi ments of a change of situation have been observed to grow on the food which it spontaneously yields, to ex ecute their accustomed motions, and, above all, to pro pagate their kind, we shall be disposed to conclude, that patient suffering has been mistaken for health, and viva ciousness for the power of accommodation.