12 



MANUAL OF THE MOLLUSCA. 



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

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

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

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

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

 banks at twenty fathoms. Deepest of all, the terebratida are ' 

 found, commonly at fifty fathoms, and sometimes at one hundred : 

 fathoms, even in Polar seas. The fairy-like pteropoda, the j 

 oceanic-snail, and multitudes of other floating molluscs, pass ! 

 their lives on the open sea, for ever out of sight of land ; whilst i 

 the litiopa and scyllcea follow the gulf-weed in its voyages, and i 

 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- i 

 predations ai'e 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 wliile that crop is on the ground.* 1 



Some, like the "cellar-snail," feed on cryptogamic vegetation, or ' 



on decaying leaves ; and the slugs aie attracted hy fungi, or any I 



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 gi-ow. Beyond fifteen fathoms, 



almost the only vegetable production is the uullipore ; but here \ 

 corals and horny zoophytes take the place of algoe and afford a 

 more nutritious diet. 



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

 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. 



