8 MANUAL OF THE MOLLUSCA. 
Other tribes retain the power of trayelling at will, and shift 
their quarters periodically, or in search of food; the riyer- 
_ mussel drags itself slowly along by protruding and contracting 
its flexible foot; the cockle and trigonia have the foot bent, 
enabling them to make short leaps; the scallop (pecten opercu- 
laris) swims rapidly by 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 downwards; the nucleobranchs 
and pteropods swim in the open sea. The cutile-fish haye a 
strange mode of walking, head downwards, on their outspread 
arms; they can also swim with their fins, or with their webbed 
arms, or by expelling the water forcibly from their branchial 
chamber; the calamary can even strike the surface of the sea 
with its tail, and dart into the air like the flying-fish.—( Owen.) 
By these means the mollusca have spread themselves over 
eyery part of the habitable globe; every region has its tribe; 
eyery situation its appropriate species; the land-snails frequent 
moist places, woods, sunny banks and rocks, climb trees, or 
burrow in the ground. The air-breathing limneids liye 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 lmpet and 
periwinkle live between tide-marks, where they are left dry 
twice a day; the trocht and purpure are found at low water, 
amongst the sea-weed; the mussel affects muddy shores, the 
cockle rejoices 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 fathoms water; scallop-banks at twenty fathoms. The 
terebratule are found at still greater depths, commonly at fifty 
fathoms, and sometimes at one hundred fathoms, eyen in Polar 
seas. The fairy-lke pteropoda, the oceanic snail, and multi- 
tudes of other floating molluscs, pass their lives on the open 
sea, for ever out of sight of land; whilst the litiopa and scyllea 
follow the gulf-weed in its yoyages, and feed upon the green 
delusive banks. 
The food of the mollusca is either vegetable, infusorial, or 
animal. All the land-snails are vyegetable-feeders, and their 
depredations 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 haye 
their likings, too, for particular plants, most of the pea-tribe 
and cabbage-tribe are fayourites, bnt they hold white mustard 
