of this Class are vigorous leapers; and of some the bounds are so vivacious, forcible and sudden, that they might almost be compared with the flight of a bird, or the shooting of a fish. The Clams or Scallops (Pecten) and their elegant relatives the Limæ, are eminent among our native species for this faculty. The mode in which the leaps are effected is always described as being the opening and sudden closing of the valves or shells; but, in the case of the former genus, I have recently found that the real organ of motion is the mantle. The edges of this being firmly closed, when the interior is filled with water, the fluid is forcibly ejected from the lips, which are relaxed for the purpose at any point according to the will of the Scallop; and by the jet of water striking on the surrounding element, the whole animal is shot to a considerable distance in the opposite direction.
Most of the MOLLUSCA are, as I have already intimated, inhabitants of the waters, and these are divided between the seas and the freshwaters, in a ratio somewhat like that which these divisions of the waters of our globe bear to each other. Of the marine kinds some dwell permanently and exclusively at the bottom of the deep sea, whence they are to be procured only by dredging. A species of Crania has been brought up from a depth of 255 fathoms. Others inhabit the open ocean, habitually or occasionally swimming on the top of the waves, or resting on the floating sea-weeds of warm latitudes. Many species confine themselves to the vicinity of the shore, where each peculiar situation and locality has its proper kinds. The sands, the ooze and mud of harbours, the boulders and loose stones of the wave-washed beach, the sides, ledges, and pools of rocks, uncovered at the recess of every tide, are all inhabited by species peculiar to the respective locality. Some species strictly marine are able to endure protracted exposure to the air, as the Periwinkle and the Limpet, the most familiar of shell-fish, which every visitor to the sea-side habitually sees clustered on the rocks close to the limits of high-water mark.
The fresh-water Mollusca manifest a similar choice of situation, though a less latitude is permitted for its exercise. Some are peculiar to large rivers, some to estuaries, others to lakes, and yet others to small ponds and ditches.
In general the habits of one species of a genus when ascertained, are found to indicate those of all its fellows of the same genus; as for instance not only is our common Cockle (Cardium edule) a burrower in the ooze at the margin of the sea, but all other species of the genus Cardium have similar habits of life. Yet Mr. Gray has enumerated a considerable catalogue of species, which break this rule, classing them under four divisions. 1st, where species of the same genus are found in more than one kind of situation, as on land, in fresh and in salt-water; 2nd, where one or more species of a genus, most of whose species inhabit fresh-water, are found in salt or brackish water; 3rd, where, on the contrary, one or more species of a genus, whose species generally inhabit the sea, are found in fresh-water; and 4th, where the same species is found both in salt and fresh-water." [9]
Of those species which dwell upon the land and breathe the air, most affect moist situations. The common Garden Snail, as is well known, retires to crevices and corners in continued dry weather, where it closes its shell with a temporary door to prevent the evaporation of its vital juices, and patiently waits the return of congenial humidity. The first shower prompts the sensitive creatures to venture abroad, and we see them crawling by dozens over our borders and garden walks, imbibing from the steaming surface the grateful moisture. The damp woods of warm countries are the situations which most reward the researches of the laborious collector of land-shells; but there are some which are found in the driest places, as stony plains, and the summits of arid hills.
GARDEN SNAIL.
Many terrestrial Mollusca which ordinarily inhabit moist places, are enabled, by a precaution similar to that adopted by our own Snail in drought, to sustain life, in such a state of retirement and suspension of their usual habits, not for a few days or weeks only, but even for many years. Numerous examples have occurred in which the land-shells of distant countries have been brought to England, alive but torpid, and have been kept shut up in drawers for twelve, eighteen, and even twenty months; manifesting no signs of life until moistened, when they presently crawled about, and began to eat. But the most singular example of this protracted sleep on record, is that of Mr. Simon's Snails, which must surely have been the veriest Rip Van Winkles among Mollusca. The following account is from the Philosophical Transactions; and the facts seem to have been carefully investigated, and well authenticated:—
"Mr. Stuckey Simon, a merchant of Dublin, whose father, a fellow of the Royal Society, and a lover of natural history, left to him a small collection of fossils and other curiosities, had among them the shells of some snails. About fifteen years after his father's death (in whose possession they continued many years), he by chance gave to his son, a child about ten years old, some of these snail-shells to play with. The boy put them into a flower-pot, which he filled with water, and the next day into a basin. Having occasion to use this, Mr. Simon observed that the animals had come out of their shells. He examined the child, who assured him that they were the same he had given him, and said he had also a few more, which he brought. Mr. Simon put one of these into water, and in an hour and a half after, observed, that it had put out its horns and body, which it moved but slowly, probably from weakness. Major Vallancy and Dr. Span were afterwards present, and saw one of the snails crawl out, the others being dead, most probably from their having remained some days in the water. Dr. Quin and Dr. Rutty also examined the living snail several different times, and were greatly pleased to see him come out of his solitary habitation after so many years' confinement. Dr. Macbride and a party of gentlemen at his house, were also witnesses of this surprising phenomenon. Dr. Macbride has thus mentioned the circumstance:—'After the shell had lain ten minutes in a glass of water that had the cold barely taken off, the snail began to appear, and in five minutes more we perceived half the body pushed out from the cavity of the shell. We then removed it into a basin, that the snail might have more scope than it had in the glass; and here, in a very short time, we saw it get above the surface of the water, and crawl up towards the edge of the basin. While it was thus moving about, with its horns erect, a fly chanced to be hovering near, and, perceiving the snail, darted down upon it. The little animal instantly withdrew itself into the shell, but as quickly came forth again, when it found the enemy had gone off. We allowed it to wander about the basin for upwards of an hour, when we returned it into a wide-mouthed phial, where Mr. Simon had lately been used to keep it. He presented me with this remarkable shell, and I observed, at twelve o'clock, as I was going to bed, that the snail was still in motion; but next morning I found it in a torpid state, sticking to the side of the glass."[10]
In treating of the food of the Mollusca, Dr. Johnston divides them into three classes; first, those which take their food in a liquid form, or suspended in water; secondly, those which are more properly carnivorous; and, thirdly, those which feed on vegetable matter.
Under the first division are comprised all those which have no distinct head, including the three classes, Tunicata, Brachiopoda and Conchifera. None of these have any power of pursuing prey, nor any organs for mastication. Yet any one who has ever examined with a microscope, either the sea-water, which appears to the naked eye pure and simple, or the impalpable sediment which lies upon the bottom, will be at no loss to discover abundant organic matter fitted to supply nutriment to these headless, generally stationary, and apparently helpless creatures. Countless millions of Infusorial animalcules sport in the clear water, altogether unappreciable by our senses, while vegetables clothed with flinty shells, the Diatomaceæœ of botanists, equally numerous and equally minute, crowd the mud on the floor of the sea.
In order that these minute bodies should afford nutriment to the headless Mollusca, a simple but effective contrivance is provided. The currents which ceaselessly play over the breathing organs, produced by the cilia which cover them, not only bring water to be respired, but come charged with the various organic particles, both animal and vegetable, that occur in the vicinity. It is, therefore, merely necessary that the
