STRUCTFEE AND PHYSIOLOGY OF THE MOLLUSCA. 



17 



simple structure, and perhaps only possesses sensibility of light 

 without the power of distinct vision. The larval bivalves have 

 also a pair of eyes in the normal position (Fig. 30) near the 

 mouth ; but their development is not continued, and the adults 

 are either eyeless, or possess merely rudimentary organs of 

 vision, in the form of black dots {ocelli) along the margin of the 

 mantle.* These supposed eyes have been detected in a great 



^ffi, 



Fig. 10. Pecten varius.^ 



many bivalves, but they are most conspicuous in the scallop, 

 which has received the name of argus from Poli on this account 

 (Fig. 10). 



In the tunicaries similar ocelli are placed between the tentacles 

 which surround the orifices. 



Sense of Hearing. In the highest cephalopods, this organ 

 consists of two cavities in the rudimentary cranium which pro- 

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



Fig. 11. Tentacle of a Nudibranch-J 



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

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

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

 in many bivalves and brachiopods. With the exception of 



* " Each possesses a cornea, lens, choroid, and nerve ; they are, without doubt, 

 organs- of vision." (Garner.) The same conclusion is amved at by Duvemoy in a 

 paper in the Annales des Sciences Naturelles for 1852. 



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

 m, the paUial cmtains ; br, the branchiae. 



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



