LAMELLIBRANCHIATA. 



333 



Sense organs. Auditory, visual, tactile, and probably olfactory 

 organs are present. The auditory organs have the form of paired 

 otocysts placed in the foot. They are apparently innervated from, 

 the pedal ganglia, or from the cerebro-pedal connective close to the 

 pedal ganglion, but the nerve fibres really arise in the cerebral ganglia. 

 They contain one or more otoliths, and are lined by hair-cells and by 

 ciliated cells. In the Protobranchiata (Fig. 258) they open to the 

 exterior by a fine canal, and contain foreign bodies (fine grains of 

 sand). 



Eyes, when present, are placed on the edge of the mantle : they 

 may either be simple pigment spots at the end of the respiratory 

 tube (Solen, Venus), or they may be more highly developed and 

 placed along the edges of the mantle in Area, Pectwiculus, Tellina, 

 and especially in Pecten and Spondylus. In the two latter genera 



FIG. 259. Openings of the siphons of Cardium edulc (from Perrier after Mobius). 



they are placed on stalks between the marginal tentacles, and have 

 an emerald-green or brown-red colour : they consist (Fig. 260) of an 

 eye-bulb with a corneal lens, choroid, iris, and a well-developed layer 

 of rods, into the external ends of which the optic nerve (from the 

 circumpallial nerve) passes. The sense of touch is specially localized 

 on the exposed parts of the body, i.e. on the edges of the mantle 

 and round the openings of the siphons. In these regions there are 

 very generally present papillae, cirri, or even tentacles (Fig. 259). 



The olfactory sense is siipposed to reside in the so-called osphra- 

 dium, which is a pigmented patch of epithelium placed one on each 

 side close behind the gills (in the roof of the supra-branchial chamber 

 near the visceral ganglia) ; it is innervated from a small ganglion 

 placed close to the visceral on the visceral commissure, in which its 

 nerve-fibres run from their origin in the cerebral ganglion. 



