238 THE LAMELLIBRANCHIA 



numerous irregularly-shaped auditory particles. In the rest of the 

 Lamellibranchia the otocysts are completely closed spherical cavities, 

 containing a number of auditory particles (otoconia) in the Fili- 

 branchia, but a single large otolith in the Eulamellibranchia and 

 Septibranchia, with the exception of Saxicava and the Anatinacea, in 

 which both otoconia and an otolith coexist in each otocyst. In 

 Ostraea both otoliths and otoconia appear to be absent. 



The cavity of the otocyst is generally lined with ciliated cells 

 (Fig. 216, e), but cilia may be absent in forms which have otoliths. 

 The nerves supplying the otocysts do not originate from the pedal 

 ganglia, but are branches of the cerebro-pedal connectives (Fig. 214, 

 o.n), and their fibres can be traced back to the cerebral ganglia. As 

 regards the physiology of the otocysts, it has been shown that 

 Lamellibranchs e.g. Anomia are able to appreciate sounds trans- 

 mitted through the water. 



Cephalic eyes in the adult state are found only in certain Fili- 

 branchia ; viz. in the Mytilidae and Avicula (Fig. 236, e). They are 

 situated at the bases of the first direct filaments of the inner gill- 

 plates, and each consists of a simple pigmented epithelial fossa which 

 contains a cuticular crystalline lens, but they do not seem to confer 

 any great sensibility on the species that possess them. Some other 

 Lamellibranchia have cephalic eyes during larval life : they are situ- 

 ated outside the velum, like the eyes of the larvae of Polyplacophora. 

 One may suppose that the mantle and the shell, which cover up the 

 whole of the body, render cephalic eyes of little use. Further, by 

 way of organic counterpoise one finds that the absence of cephalic 

 eyes is compensated by the development of analogous organs on the 

 only parts of the body that can be projected from the shell, that is 

 to say, on the edges of the mantle and the siphons. The most simple 

 arrangement consists in the presence of pigmented cells at the 

 extremities of the siphons or around the posterior pallial apertures, 

 the existence of such cells coinciding with an increased photodermatic 

 sensibility whether for both a sudden illumination and a sudden 

 obscuration as in Pholas, LitJwdomus, Mactra helvacea, and Tellina 

 complanata ; or for a sudden obscuration only, as in Ostraea and 

 certain species of Cardium and Venus ; or for an increase of illumina- 

 tion, as in Lima and Psammobia vespertina. As the result of 

 specialisation, pigment spots of this kind are transformed into 

 veritable eyes, situated on the projecting edges of the mantle and 

 siphons. 



Pallial eyes arising in this manner are of several different kinds, 

 of which the principal are those occurring in the Arcidae, Lima, 

 and the Pectinidae, Spondylus, and some species of Cardium. 



In the greater number of Arcidae, namely, in the genus Area, 

 except A. diluvii, and in Pedunculus, the pallial eyes are but little 

 differentiated in structure, but are collected in groups ; that is to say, 



