MOLLUSCA. 13 
length. The weight of the former is four pounds 
two ounces; that of the latter, four pounds nine 
ounces ; yet the snail creeps under this load at ap- 
parent ease. Nor are you much surprised when 
you see it actually in motion, for the seeming dis- 
proportion between the contained animal and con- 
taining shell has disappeared. On issuing from its 
shell, like an Hastern Genii freed from his exorcism, 
the animal has grown visibly,—has assumed a 
portlier size and more pedestrious figure. The 
body has suddenly become tumid and elastic, the 
skin and exterior organs stretched and displayed, 
the foot has grown in length and in breadth, and, 
with additional firmness, it has acquired at the 
same time the capability of being directed, bent, 
and modified in shape, to a considerable degree, as 
the surface of the road traversed may require. 
Thus it is with nearly all the cephalous mollusca ; 
and by a similar disposition of aqueducts, the foot 
of the Bivalves is equally adapted to every act sub- 
servient to their locomotion and more especially to 
the act of burrowing; for had the foot not been so 
framed as to permit of an enlargement superior to 
the size of the shell, it seems obvious that the fur- 
row could not have been made large enough to 
contain the latter. The same, too, with many 
Gasteropods which burrow in the sand when in 
search of prey. The Luccina and most carnivo- 
rous mollusca have this ability, dependent on the 
system of aqueducts we have been describing ; and 
you must observe, that from the manner in which 
the shell is attached to the body by the large re- 
tractor muscle, it so happens that this is drawn 
into the furrow always with the notch in the 
aperture uppermost, so that, when completely 
