STRUCTUEE AND PHYSIOLOGY OF THE MOLLUSCA. 



23 



Fig. 10. Peden varins* 



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 otoHthes, 

 in many bivalves and brachiopods. With the exception of 

 tritonia and eoUs, none of moUusca 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 zoophaga may be taken with animal baits. In the pearly 

 nautilus, there is a hoUow plicated process beneath each eye. 



Fig. 11. Tentacle of a Nvdibranch.% 



* Teden varins, L., from a specimen dredged by Mr. Bowerbank, off 

 Tenby ; m, the paUial cm-tains ; br, the branchioe. 



t In the Octopods, there is a foramen neai- the eye, and in some of the 

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

 oar. 



X Fig. 11. Tentacle of Foli^ coronata, Forbes, from Alder and Hancock. 



