279 CONCHIFERA. 
usually found a fleshy organ capable of being pro- 
truded beyond the limits of the shell, and of assuming 
various forms, suited to the different offices which it 
has to perform. This organ, which is small in the 
Pecten, is, in other genera, as the common Cockle, for 
example, developed to a very large size; while in 
others, as the Oyster, no trace of its presence is to be 
found. It is commonly known as the tongue or foot. 
The current which plays over the gills subserves 
also another purpose, not less necessary to lite, I 
have already said that the Bivalve Mollusca are 
unprovided with any organs for pursuing or seiz- 
ing prey; and yet their food is largely animal. 
It consists principally of those minute creatures 
belonging to the Class Jnfusoria, which, invisible 
to the unassisted eye, swarm in innumerable 
millions in the waters both of the sea and of rivers. 
The currents which pour across the gills are crowded 
with these minute creatures, and the parts that 
surround the mouth lyimg in their course, and 
being themselves also clothed with vibratory cilia, 
a portion of the stream is drawn into the gullet 
and passes into the stomach, carrying with it the 
tiny prey, on which the Mollusk fattens, without 
any exertion of its own, for the ciliary action is 
doubtless, to a considerable extent, involuntary. 
As far as is at present known all the members 
of this class are hermaphrodite; that is, the repro- 
ductive organization, instead of being assigned to 
two sexes, is perfect in every individual animal. 
In the Oyster, the ovary may be seen through the 
mantle, resting, as a whitish mass of considerable 
size, upon the muscle. It occupies the whole upper 
part of the animal, and creeps down along the sides 
and lower parts, being filled, at the time of repro- 
