Eyes of Molluscs and Arthropods. 577 



efiFect is seeu in Pecten Jacohaeus, which always rests lipon one side, 

 and wbere a maximum difference betweeu the curvature of the sbell 

 valves is accompanied by a maximum difference in the development 

 of the visual organs of the two mantle folds. But I must confess that 

 the appareutly doubtful advantag-e, gained by the eyes of the upper 

 valve, does not seem sufficient to account for the great ascendency they 

 bave gained over those of the other side ; for, unless the upper mantle 

 edge, or the stalks of the eye, are sufficiently developed to carry the 

 latter beyond the edge of the sbell, the lower mantle v^ould receive the 

 more direct rays of light from above. But the eyes of the lower mantle 

 have as long, if not longer, stalks than the upper ones, and project as 

 much beyond the edge of the sbell; so we seem to be left just wbere we 

 started from ; that is, without a sufficient reason for the greater devel- 

 opment of the eyes upon one side than upon the other. 



One finds in those species in which the eyes are especially numer- 

 ous, — Pecten vcmus, and P, opercularis^ — a number of eyes, the 

 pupils of which are entirely covered with pigment. I 

 have taken especial pains to examiue these organs, which could no 

 longer function as eyes, and bave found that the retina, with its rods 

 and nerve fibres, is as perfectly developed, as in the most perfect eyes ! 



In Pecten Jacohaeiis^ the left side of each eye stalk in the right 

 valve, as well as in the left, is the longer and therefore, when the ani- 

 mal is in its natural position, the pupi'l is directed upwards. 



The stalk of the eye has been described as colorless (except by 

 Hensen, who says that the stalk, as well as the eye itself, is covered 

 with a pigmented epithelium: ; but this is not so, for, on the upper side of 

 the eyes on the left valve, is a longitudinal band of pigment exactly 

 similar to the one over the corresponding side of the tentacles. In the 

 lower mantle fold, the bands are, morphologically speaking, on the op- 

 posite side, or on that part away from the sbell. In both cases, when 

 the animai is in its natural position, the pigmented bands are on the 

 upper side of the eye stalks, and therefore towards the light. 



The pigmented ring, around the apex of the eye, offers certain pe- 

 culiarities which bave b^retofore escaped notice, or perhaps were not 

 thought worthy of consideration. For instance, in the eyes on the median 

 part of the mantle, the cells on the upper side of the iris are completely 

 fìlled with a nearly black pigment, most deeply colored at the inner 

 ends of the cells (PI. 29, fig. 19). When seen »in toto«, this portion 

 forms a specially dark area, continuous with the pigmented band ou the 

 same side of the eye stalk. In these median portions of the mantle, the 



Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. VI. 39 



