GASTEROPODA. 307 



or recently killed animals. In the Limnceus {fig. 123.), the acoustic 



cells adhere to the posterior part of the 

 anterior ganglions of the great sub- 

 oesophageal mass {a, a)', e is the cap- 

 sule ; / the otolithes. They hold a 

 similar position in the snail and slug, in 

 which the number of otolithes ranges 

 from eighty to above a hundred. The 

 acoustic sacs are easily recognised by 

 "\ / submitting the head of the smaller 



Limnffius sta^iis specics of Gastcropod, or of the young 



of the larger species^ to a gentle com- 

 pression under the microscope. 



From the analogy of the soft mucous skin of the Gasteropods 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 MoUusca, 

 is determined by the evidence which snails afford of scenting their 

 food. 



The tongue is, in almost all the Gasteropods, a mechanical organ for 

 the attrition of the food : its complex horny uncinated 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. 



Gasteropods have the power of repairing injuries and of reproducing 

 lost parts to a considerable extent. New tentacula soon grow to 

 replace those which may have been amputated. When they support 

 eyes, as in the snail, the organs of vision are also reproduced : the 

 mouth, with the horny jaw, has grown again in this Gasteropod ; and 

 when the snail has been decapitated, but with the oesophageal gan- 

 glions left behind, the head has been restored. 



The general conditions of the sexual system have been already 

 briefly defined. The complexity and bulk of the combined organs in 

 the common slug and snail are truly extraordinary. The testis is the 

 small, compact, minutely follicular gland, imbedded in the substance 

 of the liver, and occupying, in the snail, the apex of the shell. The 

 vas deferens becomes closely attached to the oviduct in its course 

 towards the right side of the head, where it is joined by the short and 

 simple duct of a small prostatic sac in the slug, the corresponding sac 

 in the snail having a longer duct, from the middle of which a caecal 

 tube is developed. The vas deferens terminates near the base of a 

 very long and slender intromittent organ, usually retracted and con- 



X 2 



