STRUCTURE AND MYSIOLOGY OF THE MOLLUSCA. 



17 



simple pfructure, and perhaps only possesses sensibility of light 

 without the power of distinct vision. The lars'al bivalyes have 

 also a jDair of ej-es 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 



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 otoUthe 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 ; tliey are, without doubt, 

 organs of vision." {Garner.) The same conclusion is aixived at by Duvemoy in a 

 paper in the Annnles des Science!; Nahirelles for 1852. 



t Pecten varius, L„ from a specimen dredged bj' Mr. Bowerbank, off Tenby . 

 m, the pallial cui-tains ; br, the branchiae. 



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



