14 
G. H. Parker 
with the neighboriüg fibres 2 and 3, and form a fourth group. This 
arrangement explains the occurrence of groups of three and four 
retinal fibres immediately below the basement membrane (Fig. 20), 
a condition also observable in Homerus. In Palinurus^ according to 
ViALLANES (92, pag. 364), the fibres are arranged in much the same 
way as in Astacus except that there are five openings in the base- 
ment membrane instead of four, though the latter number occasionally 
occurs. 
The axis cylinder of each retinal fibre in Astacus passes as a 
transparent shaft, the fibrillar axis, through the retinular cell to which 
the fibre belongs, and disappears in the region of the rhabdome 
without extendiüg beyond the distal end of that structure (cf. Figs. 21, 
20, 19, and 24, üxm). The position at which the fibrillar axis 
disappears seems to me important as affording evidence in favor of 
the view that the rhabdome is the organ in which the optic fibres 
terminate. In an earlier paper (Parker, 90, pag. 29), I described 
a similar axis in the retinular cells of Homarus^ and I subsequently 
identified like structures in a number of crustaceans (Parker, 91, 
pag. 116), results that have since been confirmed by Viallanes 
(92, pag. 362) in his study of the eyes in Palinurus. 
In addition to the seven functional retinular cells just described, 
an eighth rudimentary cell is present in Astacus^ as in many 
and perhaps all other decapods. The nucleus of this cell is hidden 
among the nuclei of the other proximal retinular cells, and is seen 
with certainty only when these nuclei are counted. Such an 
enumeration can be carried out in a series of consecutive sections 
through a single ommatidium as shown in figures 8—13 (PI. 1). 
Here the nuclei are numbered in correspondence with the cells to 
which they belong. The most distal section (Fig. 8) contains parts 
of nuclei 1, 3, and 7, which also appear in the next section (Fig. 9) 
together with a portion of nucleus 6. Nucleus 7 extends through 
to the third section (Fig. 10), in which nucleus 6 is also represented 
and a part of nucleus 2. The fourth section (Fig. 11) contains the 
remainder of nucleus 2 and parts of nuclei 4 and 5, both of which 
reappear in the fifth section (Fig. 12). The sixth section (Fig. 13) 
contains all seven cells and the eighth nucleus in its usuai position 
on the dorsal face of cell 7; occasionally it occurs ventral to this 
cell but always associated with it. 
The body of the eighth cell seems reduced to a minimum, for 
its nucleus appears to be without surrounding protoplasm. In sec- 
