Page:Natural History, Mollusca.djvu/37

 is sufficiently slow and awkward, but some of the sand-borers are able to conceal themselves thus with surprising rapidity.

Others 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