78 ORGANS OF SENSE. 



iiuclibranchs, and Oncidium occupy vainous portions of the 

 surface of the mantle. 



The Scaphopoda are structurally degraded ; the head is rudi- 

 mentary and possesses no eyes. 



Lamellibranchiata. Some of the mollusca which in the adult 

 state are blind are provided with excellent organs of sight 

 during their more active larval existence. Thus, larval lamelli- 

 branchiates have a pair of eyes near the mouth, which are 

 wanting to the adults. However, this loss is compensated by the 

 development of numerous ocelli on the mantie-bordei\ These 

 are particularly apparent in Pecten (vi, 75), and Spondylus, 

 varying in number on the two sides of the body in the 

 inequi valve species : thus, Spondylus gaederopus has sixty ocelli 

 on the right or fixed side, ninety on the left side. Small eyes. 

 are developed also upon the tentacles or papillae sometimes 

 ornamenting the extremity of the siphons, as in Cardium edicle, 

 the UnionidiB, etc. The structure of these ocelli is similar to 

 that of the eyes in gastropods. 



Auditory Organs. The organs of hearing in mollusks consist 

 of a pair of sacks or otocysts, containing a liquid in which is 

 suspended one or several calcareous concretions termed otolites. 

 The walls of the otocysts have a vibratile, ciliated epithelium 

 causing a constant movement of the liquid which they contain. 

 A special acoustic nerve supplies the organ. 



Cephalopoda (vi, 71). In the dibranchiates the auditory sacks 

 are lodged in cavities of the cephalic cartilage : they each contain 

 a single, large, calcareous otolite. In the Nautilus, however, 

 these sacks are found attached to the pedal ganglia, and contain 

 numerous otolites. The external ears are hollow, plicated pro- 

 cesses on the side of the eyes, communicating through a passage 

 lined by a glandular membrane, with the auditory sacks. 



Gastropoda (vi, 77). Soiileyet first detected auditory organs 

 in univalves, and Siebold, Krohn, Kfilliker, Schmidt, Lacaze- 

 Duthiers and Jhering have so multiplied observations upon this 

 point, that their existence in all prosobranchiates may be con- 

 sidered highly probable. 



Two auditory vesicles usually exist, and very generally appear 

 to be sessile upon the pedal ganglia, where they appear as small 

 white points. In the heteropoda, in many nudibranchiata, as 

 shown by Hancock, and in numerous genera of branchio- and 

 pulmo-gastropoda, which have been carefully examined by 

 Lacaze-Duthiers, however, there seems to be no doubt that the 

 auditory nerves arise from the cerebral ganglia, even though the 

 vesicles ma^^ be situated close to the pedal ganglia. 



Within the vesicles are found, in many univalves a single 

 large, somewhat spherical otolite, whilst in others numerous 

 smaller ones exist, amounting to fifty, to a hundred in some 



