MUSEUM OF COMPARATIVE ZOOLOGY. 7 



The view that the cuticula and cone-cells are in contact, is strongly- 

 contrasted with that, maintained by Patten ('86, pp. 626, 642). Ac- 

 cording to this writer, the corneal cuticula is due to the activity of a 

 layer of cells, the corneal hypodermis, which intervenes between the 

 cuticula and the cone-cells. Patten has identified the corneal hypo- 

 dermis in the following genera of Decapods : Penseus, Palaemon, Pagurus, 

 and Galathea. It has also been described by Kingsley ('86, p. 863) in 

 the eyes of Crangon, and by Herrick ('86, p. 43) in the eyes of Alpheus. 

 Carriere ('89, p. 225) has recorded it in the eye of Astacus, and there is 

 now good reason for believing that a corneal hypodermis exists in the 

 eyes of all Decapods. 



Patten's statement ('86, pp. 665, 666) that the corneal hypodermis 

 u has been invariably overlooked by Grenacher," and Kingsley's asser- 

 tion ('86, p. 863) that the existence of the corneal hypodermis " was 

 utterly ignored by Grenacher," are perhaps a trifle too strong. It seems 

 much more probable that Grenacher confused the nuclei of the cone- 

 cells and corneal hypodermis. He evidently never saw both kinds of 

 nuclei in the eye of the same Decapod. In some cases he may have 

 described the nuclei of the cone-cells, in other cases those of the corneal 

 hypodermis. In both instances what he described he took to be the 

 nuclei of the cone-cells. In the eye of My sis, I believe that he ('79, 

 p. 118) described the nuclei of both the cone-cells and corneal hypoder- 

 mis, although in this case he was of the opinion that both sets of nuclei 

 belonged to the cone-cells. Where only one set is figured, it is difficult to 

 decide whether he has given the nuclei of the cone-cells or of the corneal 

 hypodermis. So far as I am aware, there are always in each ommatid*- 

 ium of a Decapod two hypodermal nuclei, and four nuclei in the cone- 

 cells. This numerical relation is sufficient to distinguish the groups of 

 nuclei, but it can only be employed satisfactorily where transverse sec- 

 tions at the proper niveau are given. Unfortunately, in the Decapods, 

 Grenacher did not figure any such sections, and it is therefore difficult 

 to decide in particular cases which kind of nuclei he has described. 



In the lobster a well differentiated corneal hypodermis has already 

 been pointed out (Fig. 1, cm. hd.). In transverse sections this presents 

 the appearance of squares of granular protoplasm (Fig. 3). Each square 

 contains two nuclei, and is bounded by a membrane. A narrow space 

 filled with granular substance separates the membranes of adjacent 

 squares. From the longitudinal section (Fig. 1) it will be seen that 

 these squares are relatively thin, so that their proportions are somewhat 

 like those of square tiles. The outer face of the tile is flat ; its inner 



