The Retina and Optic Ganglia in Pecapods, especially in Astacus. 1 5 
tions deeper than tliat in whicb nucleus 8 lies, cell 7 usually presents 
a small body, to ali appearances a fibre, eitber on its celi wall or 
occupying a deeper position apparently tbrougb Involution (Fig. 14, y], 
Whetber this body is the fìbrous end of the eighth celi 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 celi 
is present in Alpheus^ but he could not demonstrate it in this crus- 
tacean as clearly as in Palaemonetes. 
The rhabdome in Astacus is so favorable for investigation 
that I bave 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 somev^hat in detail. 
The axis of the retinula, as can be seen in thin longitudinal 
sections (Fig. 7), is occupied dislally by the cone-cells [cl.con]^ w^hich, 
however, separate after passing only a short distance into the reti- 
nula and make their v^ay as fibres around the rhabdome. The level 
at which this Separation occurs marks the distai end of a pear- 
shaped cavity [x] v^hose walls are made by the necks of the retinular 
cells and whose contents consists of a slightly coagulable fluid. This 
cavity reaches to the distai end of the spindle-shaped rhabdome, 
which, in its turn, extends almost to the basement membrane [mh.ha]. 
The relations of these different structures can be seen well in trans- 
verse sections. Figures 8 — 11, from the distai 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 bave separated, and 
the opening in the centre is the distai end of the pear-shaped cavity, 
which, however, is seen to better advantage in figure 14, where the 
fibres from the cone-cells (fbrxon) are also visible. The peculiar 
W-shaped distai 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 thronghout 
its length (cf. Figs. 17, 24, and 18). 
Although it might seem from figure 7 that the arrangement of 
the pigment at the distai 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 He on the near 
and far sides of the one process that would be effective in this 
