Eyes of Mollusca and Arthropods. 5ßl 



ones. The latter contaiu a liigbly refractive. gramüar substance, and 

 are usually couical. or wedge-sliaped, the smaller end beiug directed 

 outwards, while the widened base is turned in the opposite direction 

 and contaius a large nueleus. The latter is distingnished from the nuclei 

 of the surrouuding pigment cells, in the same way that those of the re- 

 tinophorae in HaKoiis, or in the faceted eyes of Area, are distinguished 

 from the nuclei of the retinulae. i. e. by the clearness with which they 

 stain, and the presence of a nucleolus. The nuclei of the retinulae are 

 filled with a quantity of chromatine, which, although staining deeply, 

 produces a duU and heavy efifect when contrasted with the brilliantly 

 and sharply stained nuclei of the retinophorae. This difference alone 

 would enable us to distinguish the two kinds of cells. I have sought for 

 traces of a second nueleus in the retinophorae, but in vain ; this is pro- 

 bably due to their small size. and the difficulty of macerating them prop- 

 erly. On account of the paucity of these cells, they may be easily over- 

 looked, or confounded with colorless ones of the surrounding epithelium. 

 These diffieulties, coupled with the fact that under the most favorable 

 circumstances the aborted nueleus is very hard to observe, render it more 

 than probable that such a nueleus has been overlooked, rather than that 

 it is not present at all. The number of these cells is comparatively small; 

 I have never seen, in sections of one eye, more than three. Looking into 

 the eye from above, on surface preparations, six or seven colorless cells 

 are seen as bright, refractive spots, while it is probable that there are two 

 or three more, that could not be seen, on the sides of the pit ; therefore a 

 typical invaginated eye would consist of from seven to ten ommatidia. 



The remainder of the retinal layer is composed of large, cylin- 

 drical cells filled with dark brown, or in some cases nearly black pig- 

 ment. Only their bases are colorless, so that one can see the lower 

 portion of the deeply stained nuclei, the outer parts being closely envel- 

 oped in pigment (PL 30, fig. 49). 



The nuclei of both pigmented and colorless cells are so deeply situ- 

 ated that they seem, in some cases, to rest upon the basal membrane, 

 which is hardly more developed bere than below the ordinary epi- 

 thelium (fig. 42). 



Below, and parallel to the basal membrane, is a number of con- 

 nective tissue fibres, besides many large gland cells with refractive and 

 granular Contents, and excentrically placed nuclei. 



The cuticula — hearing in mind that it is by no means certain that 

 all the structures with this name are homologous, or chemically ident- 

 ical — is well developed over all the pigmented cells, while it becomes 



Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. VI. 38 



