HABITS AND ECONOMY OF THE MOLLUSCA. 11 



sedentary tribes settle in the place they intend to occupy during 

 the remainder of their lives. The tunicary cements itself to rock 

 or sea-weed ; the shipworm adheres to timber, and the pholas 

 and UtJiodomus to limestone rocks, in which they soon excavate 

 a chamber which renders their first means of anchorage unneces- 

 sary. The mya and razor-fish burrow in sand or mud; the 

 mussel and pinna spin a byssus ; the oyster and spondylus 

 attach themselves by spines or leafy expansions of their shell ; 

 the brachiopoda are all fixed by similar means, and even some 

 of the gasteropods become voluntary prisoners, as the Upponyx 

 and vermetus. 



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 leaps ; the scallop (pector opercularis) 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- down wards ; the nucleobranches and 

 pteropods swim in the open sea. The cuttle-fishes 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 air 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, or woods, or sunny banks and rocks, climb trees, 

 or burrow in the ground. The air-breathing limneids live ir 

 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 



