174 SYSTEM OF AQUEDUCTS. 



osa, which measures fully ten inches in length, and upwards 

 of eight in breadth ; another of S trombus gigas is nearly 

 one foot in 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 apparent ease. Nor 

 are you much surprised when you see it actually in motion, 

 for the seeming disproportion between the contained animal 

 and containing shell has disappeared. On issuing from its 

 cell, like an Eastern 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 subservient 

 to their locomotion, and more especially to the act of burrow- 

 ing ; 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 furrow 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 

 Buccina and most carnivorous mollusca have this ability, 

 dependant on the system of aqueducts we have been de- 

 scribing ; and you must observe, that from the manner in 

 which the shell is attached to the body by the large retractor 

 muscle, it so happens that this is drawn into the furrow 

 always with the notch in the aperture uppermost, so that, 

 when completely buried, the animal is still enabled to com- 

 municate with the water by its respiratory siphon. 



