MOLLUSC A— THE SENSORY ORGANS 



175 



In Pedcn, the optic nerve wliicli runs to eacli eye from the nerve for the pallial 

 edge, divides, close to the eye, into two branches. One of these rnns to tlie base of 

 the optic [ilate, and tliere l)reaks up into fibres, which radiate on all sides to the edge 

 of the plate, then bend over towards the retina to innervate some of its cells. The 

 other branch runs direct to the edge of the plate, there bends round at a right angle 

 and supplies nerves to the rest of the nerve cells. The fibres of this branch are not, 

 however, directly connected with the retinal or rod cells, as there is a layer of anas- 

 tomosing ganglion cells interposed between the two. Between the pigmented epi- 

 thelium and the rod layer of the retina, a tapetum lucidum is found, which gives the 

 eye of the Pedcn its metallic lusti'e. 



Dorsal eyes are found in many species of Oacidium. They lie at the tips of the 

 contractile papillre found on the dorsal integument of this curious Pulvwnate ; on each 

 papilla three or four such eyes occur. Besides these, Oncidium has the two normal 

 cephalic eyes usually found in Gastropods. 



The pallial eyes of the Lamellihranchiates, Pecten and Spondtjlus, are found in 

 large numbers on the edge of the mantle, between the longer tentacles, and on the 

 tips of shorter tentacles. The rods of the retina in Pcden, when fresh, are of a very 

 evanescent red colour (visual purple ?). 



.'). The Eyes on the Shell of Chiton. 



These have already been described (p. 167). Their morphological significance 

 cannot be determined as long as their development is unknown and their histological 

 structure imperfectly investigated. 



6. The Compound Eyes of Area (Fig. 151) and Peetunculus. 



These are found in great numbers at the edge of the mantle, and 

 are epithelial organs which do not in any way agree in structure with 

 the other visual organs found in 

 Mollusca, but rather resemble 

 certain simple Arthropodan 

 eyes. 



In form they resemble an 

 externally convex shell. The 

 unilaminar epithelial wall of 

 the shell passes, at its edge, 

 into the surrounding pallial 

 epithelium. In section, its com- 

 ponent elements appear to be 

 arranged like a fan ("Fjicher- 

 auge"). These elements are of .. ,., oo„t^„ ^r tv.,. <>„<. „f a».^, v,ov>,otQ 



° ' , . . I to. lal.— Section of the eye of Area Daroata 



three kinds : (1) conical visual (adapted from Rawitz). l, Retinal cellswitUrod-liko 

 cells, with their bases turned bodies (2); 3, i.igment cells; 4, slender interstitial 



outwards; (2) a sheath of six 



cylindrical pigment shells surrounding each visual cell. Each group, 

 consisting of one visual cell and its surrounding pigment cells, may be 

 considered as a single eye or ommatidium of the simplest structure, in 

 which the retinula is represented by one single visual cell. (3) Slender, 

 almost thread-like interstitial cells which stand between the ommatidia. 



