8 MANUAL OF THE MOLLUSCA. 



Other tribes retain the power of travelling at will, and shift 

 their quarters periodically, or in search of food; the river- 

 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 leaj)s ; the scallop (joeden 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 cuttle-fish have 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 ah' like the flying-fish. — {Owen.) 



By these means the mollusca have spread themselves over 

 every part of the habitable globe ; every region has its tribe ; 

 every 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 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 inirpurce are found at low water, 

 amongst the sea- weed ; the mussel afi'ects 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 

 terehratulce are found at still greater depths, commonly at fifty 

 fathoms, and sometimes at one hundred fathoms, even in Polar 

 seas. The fairy-like 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 scylloea 

 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 kno'vsTi to the gardener and 

 farmer ; many a crojD 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, bnt they hold vrhite mustard 



