MOLLUSCA. 
489 
foot ; the cockle and trigonia have the foot bent, enabhng them to make short leaps ; the scallop 
swims rapidly hj opening and shutting its tinted valves. Nearly all the gasteropods creep like 
the snail, though some are much more active than others ; the pond-snails can glide along the 
surface of the water, shell-downward; the nucleobi'anches and pteropods swim in the open sea. 
The cuttle-fishes have a strange mode of 
walking, head-downward, on their out- 
spread arms; they can also swim with 
their fins, or with their webbed arms, or 
by expelling the water forcibly from their 
branchial chamber; one species of cala- 
mary can even strike the surface of the 
sea with its tail, and dart into the air like 
the flying-fish. 
By these means the molluscahave spread 
themselves over every part of the habit- 
able globe; every region has its tribe, 
every situation its appropriate species ; 
the land-snails frequent moist places, or 
woods, or sunny banks and rocks, climb 
trees, or burrow in the ground. The air- 
-breathing limneids live in fresh water, only coming occasionally to the surface ; and the auriculas 
live on the sea-shore, or in salt marshes. In the sea each zone of depth has its molluscous 
fauna. The limpet and periwinkle live between tide-marks, where they are left dry twice a day ; 
the trochi and purpurse are found at low water among the sea-weed ; the mussel aftects muddy 
shores ; the cockle rejoices in extensive sandy flats. Most of the finely-colored shells of the tropics 
are found in shallow water, or among the breakers. Oyster-banks are usually in three or four 
fathom water ; scallop-banks at twenty fathoms. Deepest of all the terebratulas are found, com- 
monly at fifty fathoms, and sometimes at one hundred fathoms, even in polar seas. The fairy- 
like pteropoda, the oceanic snail, and multitudes of other floating molluscs, pass their lives on the 
open sea, forever out of sight of land ; while the lisiopa and scyllsea follow the gulf-weed in its 
voyages, and feed upon the green delusive banks. 
^ The food of the mollusca is either vegetable, infusorial, or animal. All the land-snails are 
vegetable-feeders, and their depredations are but too Avell known to the gardener and farmer ; 
many a crop has been wasted by the ravages of the small gray slug. They have their likings, 
too, for particular plants : most of the pea-tribe and cabbage-tribe are favorites, but they hold 
white mustard in abhorrence, and fast or shift their quarters while that crop is on the ground. 
Some, like the cellar-snail, feed on cryptogamic vegetation, or on decaying leaves; and the slugs 
are attracted by fungi, or any odorous substances. The round-mouthed sea-snails are nearly all 
vegetarians, and are consequently limited to the shore and the shallow waters in Avhich sea-weeds 
groAv. Beyond fifteen fathoms, almost the only vegetable production is the nuHipore ; but here 
corals and horny zoophytes take the place of algse, and afford a more nutritious diet. 
The whole of the bivalves, and other headless shell-fish, live on infusoria, or on microscopic 
vegetables, brought to them by the current which their ciliary apparatus perpetually excites ; 
such, too, must be the sustenance of the magilus, sunk in its coral bed, and of the calyptr^ea, 
fettered to its birth-place by its calcareous foot. 
The carnivorous tribes prey chiefly on other shell-fish, or on zoophytes, since, with the excep- 
tion of the cuttle-fishes, their organization scarcely adapts them for pursuing and destroying 
other classes of animals. One remarkable exception is formed by the stylina, which lives para- 
sitically on the star-fish and sea-urchin ; and another by the testacelle, which preys on the com- 
mon earth-worm, following it in its burrow, and wearing a buckler, which protects it in the rear. 
Most of the siphonated univalves are animal-feeders ; the carrion-eating stromb and whelk con- 
sume the fishes and other creatures whose remains are always plentiful on rough and rocky 
coasts. Many wage war on their ow^n relatives, and take them by assault ; the bivalve may close, 
Vol. IL— 62 
