MOLLUSC A. 



489 



THE POULPB OK CUTTLE-FISH. 



foot; the cockle and trigonia have the foot bent, enabhng them to make short leaps; the scallop 

 swims rapidly by opening and shutting its tinted valves. Neatly 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 watei', shell-downward; the nucleobranches and pteropods swim in the open sea. 



The cuttle-fishes have a strange mode of 

 walking, head-downwaid, 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. Li the sea each zone of dej)th has its molluscous 

 fauna. The limpet and periwinkle live between tide-marks, where they are left dry twice a day; 

 the trochi and purpurjB are found at low water among the sea-Aveed ; the mussel affects 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 bi'eakei's. Oyster-banks are usually in three or four 

 fathom water ; scallop-banks at twenty fathoms. Deepest of all the terebratulje are found, com- 

 monly at fifty fathoms, and sometimes at one hundred fathoms, even in polar seas. The fiury- 

 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 tlie lisiopa and scylhiea 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 well known to the gardener and farmer ; 

 many a crop lias been wasted by the ravages of the small gray slug. They have their likings, 

 too, for particular plants : most of the pea-ti'ibe and cabbage-tribe are favorites, but they hold 

 white mustard in abhorrence, and fast or shift their quarters wdiile that crop is on the ground. 

 Some, like the cellar-snail, feed on cryptogamic vegetation, or on decaying leaves; and the slugs 

 arc 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 which sea-weeds 

 grow. Beyond fifteen fathoms, almost the only vegetable production is the nullipore; but here 

 corals and horny zoophvtes take the place of algaj, 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 calyptra^a, 

 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 own relatives, and take them by assault ; the bivalve may close, 

 Vol. IL— 62 



