FOOD OF 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- 
si.e polyps, bryozoa, and ascidians, 
for the waters of the ocean har- 
bour such inealeulable multi- 
tudes of microscopic animals and 
plants that their moderate ap- 
petite never remains long un- 
satisfied. The same streams 
: - k / 
which aérate their blood also Pinna. 
eonvey) to thei mouth all the» orcucs fom, which; the Slamonts ane 
d. Inferior base of the foot. 
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 tke spot, would 
