MOLLUSCA. 25 
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 re- 
latives the Lime, 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 re- 
cently 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 Mouiusca 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 ex- 
clusively 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 con- 
fine themselves to the vicinity of the shore, where 
