STRUCTURE AND PHYSIOLOGY OP THE MOLLUSCA. 23 



Pig. 10. Pecten varius.* 



Sense of Hearing. In the highest cephalopods, this organ 

 consists of two cavities in the rudimentary cranium which pro- 

 tects the brain ; a small calcarious body or otolithe is suspended 

 in each, as in the vestibular cavities of fishes. f Similar auditory 

 capsules occur near the base of the tentacles in the gasteropoda, 

 and they have been detected, by the vibration of the otolithes, 

 in many bivalves and brachiopods. With the exception of 

 tritonia and eolis, none of mollusca have been observed to emit 

 sounds. (Grant). 



Sense of Smell, This faculty is evidently possessed by the 

 cuttle-fishes and gasteropods ; snails discriminate their food by 

 it, slugs are attracted by offensive odours, and many of the ma- 

 rine zoopliaga, may be taken with animal baits. In the pearly 

 nautilus, there is a hollow plicated process beneath each eye, 



Fig. 11. Tentacle of a Nudibranch.% 



* Pecten varius, L., from a specimen dredged by Mr. Bowerbank, off 

 Tenby ; m t the pallial curtains ; 5r, the branchice. 



f In the Octopods, there is a foramen near the eye, and in some of the 

 Calamaries a plicated organ, which M. D'Orbigny regards as an external 

 ear. 



$ Fig. 11. Tentacle of Eolis eoronata> Forbes, from Alder and Haneock. 



