No. I.] EYES OF MOLLUSCS AND ARTHROPODS. 8 1 



papillae, from each one of which arises an enormously long 

 cilium. 



The sense-hair papillcB , which originate at any place along the 

 outer surface of the velum, first appear as thickenings of the 

 hypodermis which soon become conical, and provided with a 

 tuft of stiff sense-hairs at the apex. The inward proliferation of 

 the cells at the apex of the thickening gives rise to an ectoder- 

 mic core, which becomes transformed into the longitudinal 

 nerve with which every tentacle is provided. As the papillae 

 increase in length, tufts of sense-hairs are formed on the sides, 

 each connected with one or two ganglionic cells. In those 

 papillae which do not develop into tentacles no distinct nerve is 

 formed ; but two or three cells separate from the summit 

 of the papilla and wander into the underlying tissue, there 

 forming ganglionic cells, the nerve-like ends of which may ter- 

 minate in a small number of sense-hairs ; or, if the cells are more 

 highly specialized, the sense-hairs may be absent, while the ter- 

 minal fibres divide into numerous fibrillae which supply the 

 adjacent cells. 



We have found the same sensitiveness to changes in the inten- 

 sity of light in Ostrea, Mactra stultorum, M. solidissima, Pinna, 

 and Avicnla, that was so marked in Area. In these cases, how- 

 ever, there were no well-defined eyes, but large pigmented and 

 shallow grooves, or slightly depressed areas over which the 

 cuticula was but little thickened. In these pigmented patches 

 were numerous ommatidia having essentially the same construc- 

 tion as those in the pseudo-lenticulate and invaginate eyes of 

 Area. They undoubtedly belong to the simplest of light-sensi- 

 tive organs ; but, in spite of their simple structure, they are in 

 some cases wonderfully delicate organs. In Avicula, for 

 instance, the simple and diffuse ommatidia, the only visual 

 organs present, are able to perceive the difference in light pro- 

 duced by holding such a small object as a pencil between them 

 and the source of light. Under such circumstances a shadow 

 so faint as to be imperceptible to the experimentor caused im- 

 mediate contractions of the whole animal. 



At the end of each siphonal tentacle of Cardium edule, 

 beneath a semicircular band of pigment is a minute eye. It 



