FOOD OP BIVALVES. 



305 



factured into gloves and other articles of dress, though more as 

 an object of curiosity than for use. 



Thus we find in the same class of animals the same organ 

 most variously modified in form and structure ; now serving 

 as a foot, now as a spade, or as a rasp, or as a spinning machine, 

 and, throughout all these modifications, admirably adapted in 

 every case to the mode of life 

 of its possessor. 



The whole construction, and 

 generally the extremely restricted 

 locomotion, of the bivalves tells 

 us at once that they are unable 

 to attack their prey, but must be 

 satisfied with the food which the 

 sea-currents bring to the door of 

 their shells, or within the vortex 

 of their branchial siphons. But 

 they have as little reason to com- 

 plain as the equally slow or ses- 

 sLe polyps, bryozoa, and ascidians, 

 for the waters of the ocean har- 

 bour such incalculable multi- 

 tudes of microscopic animals and 

 plants that their moderate ap- 

 petite never remains long un- 

 satisfied. The same streams 

 which aerate their blood also 

 convey to their mouth all the 

 food which they require. 



Deprived of more active weapons, most bivalves rely upon 

 their shells as their best means of defence, and to answer this 

 purpose, their stony covering must naturally increase in solidity 

 the more its owner is exposed to injury. The pholades, litho- 

 domes, and teredines, which scoop out their dwellings in stone 

 or wood, and thus enjoy the protection of a retrenched camp, 

 can do with a thin and brittle or even with a mere rudimentary 

 shell. The solens, which at the least alarm bury themselves 

 deeper and deeper in the sand, likewise require no closely-fitting 

 valves ; but the oysters or mussels, which have no external 

 fortress to retire to, and are unable to move from the spot, would 



Pinna. 



c. Pedicle from which the filaments are 



detached. 



d. Inferior base of the foot. 



