PTEROPOt»A AND GASTROPODA. 553 



water, were doubtless intended to be heard by others of the same 

 species. The very general existence of an acoustic apparatus under 

 its most simple conditions, in the lower MoUusks, has been established 

 by the discoveries of Siebold. It consists of two round vesicles, 

 containing fluid and crystalline or elliptical calcareous particles, or 

 otolites, remarkable for their oscillatory action in the living or 

 recently killed animals. In the LimntBUs {Jig- 202.), the acoustic 

 cells adhere to the posterior part of the 202 



anterior ganglions of the great sub-ceso- 

 phageal mass {a a): e is the capsule ; / 

 the otolites. They hold a similar position 

 in the snail and slug, in which the num- 

 ber of otolites ranges from eighty to above 

 a hundred. The acoustic sacs are easily 

 recognised by submitting the head of the 

 smaller species of Gastropod, or of the 

 young of the larger species, to a gentle 

 compression under the microscope. The Limnaeus stagnabs. 



movements of the otolites, due to the action of vibratile cilia, is 

 truly, as Siebold remarks, " a wonderful spectacle." 



From the analogy of the soft mucous skin of the Gastropods to 

 the pituitary membrane of the nose, Cuvier was led to conjecture 

 that it might be the seat of the sense of smell ; but the analogy seems 

 to be too vague to render so general a diffusion of the nerves of a 

 special sense very probable. That the sense is possessed by these 

 Mollusca, is determined by the evidence which snails afford of scent- 

 ing their food : the structure which best suggests the olfactory 

 function is that which the two conical tentacles present in Doris, 

 Tritonia, and Scyllcea. The confluent tentacles forming the cephalic 

 lobe in the Bullicl^ may have the same function.* It is remarkable 

 that the laminated dorsal tentacles of the Nudibranchs, which seem 

 never to be used as organs of touch, are supplied with nerves from 

 the fore part of the super-oesophageal ganglions. 



The tongue is, in almost all the Gastropods, a mechanical organ 

 for the attrition of the food : its complex horny unciuated armature 

 seems to unfit it for the delicate office of appreciating the sapid 

 qualities of nutritive substances ; but some sense of taste may be 

 exercised by the soft membranes of the pharynx. 



The Gastropods are organised to subsist on a great variety of 

 food : they select both animal and vegetable matter in both their 

 living and decomposing states. The damage which the common slug 



* CCCXLIX. p. 189. 



