Eycs of Molluscs and Arthropods. (515 



Among- the mauy autliors , wlio liave describcd , iu more or less 

 detail, tlie eyes of Molluscs, tliree liave given special attention to this 

 genus. Tliere is uo room for a g-reat diöerence of opinion as to the 

 coarser auatomy of the eye. It is principally concerning the more min- 

 ute, histologieal structuvc that I desire to speak, and to which I have 

 giveu most attention. There is hardly any difference of moment be- 

 tween the eyes oi Haliotis, and the invaginated ones foimd in Area. 

 The former may be cousidered as pigmented pits in the thickened 

 hypodermis, with a specially thick cuticnlar covering, the outer and 

 inner layers of which have become highly differentiated to form, in the 

 first instance, the vitreous body and the lens, and, in the second, 

 the richly innervated layer of rods. The so-called retinal cells eon- 

 sist of two elements, exactly homologous with the pigmented cover cells 

 and the retinophorae of Area. The pigmented cells are extremely long 

 and narrow (figs. 68 and 62) , their inner third or half being reduced to 

 a slender hyaline stalk, or bacillus. The leugth of the cells varies 

 greatly, those opposite the opening of the cup being the longest. The 

 nuclei form a gentle swelling in about the middle of the cells, they may, 

 however, be situated at different levels, so that it might easily be im- 

 agined there were two nuclei in each cell. Such, however, is not the 

 case, as it may easily be seen that the outer ends of the isolated pig- 

 ment cells are completely filled with inteusely black pigment with which 

 they seem to terminate ; the clear, central axis of Carrière has no exis- 

 tence. In special preparations , the cell is seen to be sharply con- 

 stricted at its outer end luto a colorless rod, narrow at the base, and 

 continued outwards until it terminates in an expanded end (fig. 62 rh). 

 The pigment contained in these cells usually cousists of fine , dark 

 granules which often fuse to form large , round balls, equal in dia- 

 meter to the width of the cell. 



The bacilli (figs. 68 and 62 he.) terminate at their inner ends in 

 severa! fine fibres, which appear to restupon a very delicate basal 

 membrane. Several of these cells, the exact number I have been unable 

 to determiue , surround a single , colorless one with a large basal uu- 

 cleus ; the colorless ones are the so-called «Stützzellen«, or secrete-cells, 

 as some have considered them, but they will here be called the retino- 

 phorae, siuce they are homologous with the similarly named cells in 

 the eyes of Area and Pecten ; although, as I have already remarked in 

 the introduction, the pigment cells likewise support retinidia, still the 

 colorless cells alone perform this function in the more highly develop- 

 ed forms. Just as in Area and Peeten^ the retinophorae oi Haliotis 



