12 MANUAL OF THE MOLLUSCA. 



a-day ; the trochi and purpura are found at low water, amongst 

 the sea-weed ; the mussel affects muddy shores, the cockle re- 

 joices in extensive sandy flats. Most of the finely-coloured shells 

 of the tropics are found in shallow water, or amongst the breakers. 

 Oyster-banks are usually in four or five fathom water ; scallop- 

 banks at twenty fathoms. Deepest of all, the terebratula are 

 found, commonly 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, for ever out of sight of land ; whilst 

 the litiopa and scylltea 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 de- 

 predations are but too well known to the gardener and farmer ; 

 many a crop of winter corn and spring tares has been wasted by 

 the ravages of the " small grey slug." They have their likings, 

 too, for particular plants, most of the pea-tribe and cabbage- 

 tribe are favourites, but they hold white mustard in abhorence, 

 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 \^ fungi, or any 

 odorous substances. The round-mouthed sea-snails are nearly 

 all vegetarians, and 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 zoophytes take the place of algce and afford a 

 more nutritious diet. 



The whole of the bivalves, and other head-less shell-fish, live 

 on infusoria, or on microscropic 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 



* Dilute lime-water and very weak alkaline solutions are more fatal to 

 snails than even salt. 



