320 CONCHIFERA. 



very muscular and sensitive, being provided with circular 

 fibres for its protrusion, and with longitudinal muscular 

 bands for its retraction. Some of the Conchifera, as the 

 Trigonias, Cockles, and Venuses, move about on the surface 

 of the sand by placing their bent foot under their shells and 

 suddenly straightening it ; some, again, bury themselves in 

 the mud and sand by means of their long, conical foot, as 

 the Eazor-fish, the Gapers, and the fresh-water Pearl-mus- 

 sels ; some, again, are loosely attached to submarine rocks by 

 a long byssus, as the Mussels and Pinnas, in which case 

 the foot is rudimentary, and furnished with a gland which 

 secretes the tenacious filaments of the byssus ; others per- 

 forate stones and rocks, as the Pholades, in which the foot 

 is strengthened for the purpose with silicious granules ; 

 others burrow in wood, as the Teredines or Ship-worms, 

 which, on this account, are sometimes very destructive, 

 doing much mischief to timber in dockyards. Many 

 Bivalves, on the other hand, swim freely about by alter- 

 nately expanding and closing the valves of their shell, as the 

 Pectens, which have hence been termed the " butterflies of 

 the deep ;" others are affixed by the surface of the valves to 

 foreign bodies at the bottom of the sea, as the JEtheria and 

 Ostrea, in which case the foot, in the adult, being no longer 

 required, is obsolete or absent, or it is present only in the 

 young state, when the animal moves freely about. The 

 Arcs crawl, like Gasteropods, upon a locomotive disk, and 

 easily ascend perpendicular surfaces by means of their ex- 

 panded foot. 



The soft bodies of these animals are protected by shelly 

 valves, which are placed one on each side, and which usually 

 assume the form of hollow, flattened cones. They are 

 variously coloured and sculptured on their outer surface, 



