ORGANS OF SENSE. 



505 



many distinct eyes thus singularly situated ; and, from the circum- 

 stance of their being furnished with so many organs of vision, he applied 

 the name of Argus to the mollusca possessing them. Should the brilliant 

 specks in question be really ocelli, they certainly are placed in the only 

 position where they can be efficient as instruments of sight, inasmuch 

 as the margin of the mantle is, in such animals, the only portion of 

 the body capable of being protruded beyond the boundaries of the shell 

 to a sufficient distance to allow the creature to peep into the world 

 around it. 



(1330.) The elaborate researches of M. Siebold have demonstrated 

 the existence of another sense in the Conchiferous Mollusca, namely 

 that of hearing or at 



least have pointed out Fig. 252. 



the presence of an 

 organ which from its 

 structure seems to be 

 appropriated to the 

 reception of sonorous 

 impressions. This re- 

 markable apparatus is 

 situated in the foot, 

 and is thus described* 

 by its discoverer as 

 it occurs in Cyclas 

 cornea : " On com- 

 pressing the extre- 

 mity of the foot of 

 this species between 

 two plates of glass, 

 we bring into view a 

 large central nervous 

 ganglion, and on each 

 side of this there is a 

 minute round reser- 

 voir, composed of an 



elastic, Opake, and Cardium: a, oral orifice; 6, foot. 



tenacious substance. In the centre of this is contained a perfectly 

 transparent circular and flattened nucleus, which floats disconnected 

 from the sides of the body that contains it, and has an oscillatory move- 

 ment. This nucleus appears to consist of a crystalline salt." 



(1331.) The ovary is generally a wide glandular sacculus, occupying 



a considerable portion of the visceral mass. In the Oyster, when full of 



spawn, it is largely spread through the body ; and if at such seasons 



its delicate walls are ruptured, countless ova of microscopic dimensions 



* Ann. des Sci. Nat. n. s. torn. ix. 



