The Retina and Optic Ganglia in Decapods, especially in Astacus. 1 5 
tions deeper than that in which nucleus 8 lies, cell 7 usually presents 
a small body, to all appearances a fibre, eitber on its cell wall or 
occupying a deeper position apparently through Involution (Fig. 14, y). 
Whether this body is the fibrous end of the eighth cell or not, I 
am unable to state with certainty, but its position and numerical 
relations favor this view. 
Herrick (92, pag. 446) believes that the eighth retinular cell 
is present in Älpheus^ but he could not demonstrate it in this crus- 
tacean as clearly as in Palaemonetes. 
The rhab dorne in Astacus is so favorable for investigation 
that I have made it the subject of rather careful study, hoping 
thereby to gain a clearer insight into its composition. I shall there- 
fore describe it and its surrounding structures somewhat in detail. 
The axis of the retinula, as can be seen in thin longitudinal 
sections (Fig. 7), is occupied distally by the cone-cells [cLcon], which, 
however, separate after passing only a short distance into the reti- 
nula and make their way as fibres around the rhabdome. The level 
at which this Separation occurs marks the distal end of a pear- 
shaped cavity {x) whose walls are made by the necks of the retinular 
cells and whose contents consists of a slightly coagulable fluid. This 
cavity reaches to the distal end of the spindle - shaped rhabdome, 
which, in its turn, extends almost to the basement membrane [mb, ha]. 
The relations of these different structures can be seen well in trans- 
verse sections. Figures 8 — 11, from the distal portion of the retinula, 
Show the cone-cells in the axis of this structure ; at the level repre- 
sented by figures 12 and 13, the four cone-cells have separated, and 
the opening in the centre is the distal end of the pear-shaped cavity, 
which, however, is seen to better advantage in figure 14, where the 
fibres from the cone-cells [fbr.con] are also visible. The peculiar 
W-shaped distal end of the rhabdome is seen in figure 16; this 
expands rapidly to the squarish form characteristic of most trans- 
verse sections of this organ (Fig. 24) and retained by it nearly to 
its blunt proximal end (Fig. 18). The relative position of the cells 
and fibres that surround the rhabdome remains the same throughout 
its length (cf. Figs. 17, 24, and 18). 
Although it might seem from figure 7 that the arrangement of 
the pigment at the distal end of the rhabdome would prevent the 
direct entrance of light into that structure, such is not the case; for, 
as can be seen in figure 16, portions of the rhabdome lie on the near 
and far sides of the one process that would be effective in this 
