110 BULLETIN OF THE 



Fig. 134) figure of Astacus, in which each facet is suhdivided hy two 

 diagonal lines into four areas, and Newton's ('73, p. 327) statement 

 that the same condition occurs in Homarus, are probably incorrect. 



The cones in Decapods are composed of four segments. This number 

 was first observed by Will ('40, p. 13) in Palsemon, and has since been 

 recorded in many other genera. So far as I am aware, there are no 

 Decapods in which the number of segments is not four. As Claparede 

 ('60, p. 194) first pointed out in Galathea and Pagurus, each segment 

 contains a nucleus and represents a single cell. Although the signifi- 

 cance of these nuclei was without doubt first fully appreciated by 

 Claparede, it is probable that they were previously seeu by Leydig 

 ('55, Taf. XVII. Fig. 31) in the crayfish. 



As a rule, the distal termination of the cone cells is on the proximal 

 side of the corneal hypodermis. In the lobster, however, and in Palae- 

 monetes (Plate IX. Fig. 104), the pointed ends of these cells pass 

 between the two cells of the corneal hypodermis, and probably come 

 in contact with the corneal cuticula near the middle of a facet. 



It is difficult to determine with accuracy the proximal termination of 

 the cone cells. They can be easily traced to a region immediately distal 

 to the distal end of the rhabdome. In this region, as Schultze ('G8, 

 Taf. I. Figs. 9 and 11) has clearly demonstrated in Astacus, the fibrous 

 ends of the four cone cells separate, and pass partially around the rhab- 

 dome. In Homarus, these fibres extend proximally, and finally ter- 

 minate at the basement membrane. A similar method of termination 

 also occurs in Palinurus. In the other genera which I have studied, the 

 fibres, although visible near the distal end of the rhabdome, are lost in 

 the adjacent tissue, and I do not know whether they terminate in this 

 tissue without special attachment, or whether they make their way as 

 excessively fine fibres to the basement membrane. The separation of 

 the fibrous ends of the cone cells, near the distal end of the rhabdome, 

 has been observed by Steinlin ('GG, p. 93) in Palrcmon, and by Schultze 

 ('G7 and '68) in several other Decapods. The statement made by many 

 of the older investigators, and recently reaffirmed by Patten, that the 

 cone and rhabdome are parts of one continuous structure, is without 

 doubt incorrect. 



The resolution of the retinula into its cellular constituents was first 

 attempted in Decapods by Leydig ('55, p. 408), according to whom the 

 retinula of Herbstia contains four cellular bodies, the nuclei of wdiich 

 can be distinguished in the distal part of the structure. A somewhat 

 similar condition was described by Newton ('73, p. 333) for Homarus ; 



