HABITS AND ECONOMY OF THE MOLLUSCA. 135 



valves, and the larger species are thus able to make leaps of one 

 or two 3^ards ; the nearly related Lima vaults through the water 

 like the butterfly through the air. — (Moquin-Tandon.) Nearly 

 all the gastropods creep like the snail, though some are much 

 more active than others ; tlie 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; 

 the}^ 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 frequ^ent 

 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 the tide-marks, where they are left dry twice a day ; 

 the Trochus and Purpura 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-colored shells of 

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

 Oj^ster-banks are usually in four or five fathoms water ; scallop- 

 banks at twenty fathoms. The Terebratulse are found at still 

 greater depths, commonly at fifty fathoms, and sometimes at one 

 hundred fathoms or more, even in Polar seas. The fairy -like 

 Pteropoda, the oceanic snail, and multitudes of other floating 

 mollusks, pass their lives on the open sea, forever out of sight 

 of land ; whilst the Litiopa and Scyllsea follow the gulf-weed 

 in its voyages, and feed upon the green delusive banks." — 

 (Woodward.) 



Parasitisyn is rare among mollusks, and only exists in con- 

 nection with the Echinodermata and Coelenterata. The best 

 known parasitic genus is Stylifer, a little gastropod, numerous 

 in species. S. Turtoni attaches itself to the spines surrounding 

 the anus of different British species of Echinus (sea-urchins) ; 

 S. adericola of the Galapagos Islands digs itself a comfortable 

 dwelling in the tissues of an Asterias (Cuming); S. Orhignyi 

 lodges itself in the spines of Cidaris, causing a malformation. 

 Some of the nearly-related genus Styliferina live on Ophiura, 

 and certain species of Eulima in Holothuria. Entoctoncha mira- 

 bilis is found floating freely in the body of Synapta, in which it 

 develops its little operculated shells. J. Miiller, who discovered 

 this curious parasite, supposed that there existed in Synapta an 



