14 
G. H. Parker 
with the neighboring fibres 2 and 3, and form a fourtb gronp. Tbis 
arrangement explains tbe occurrence of groups of tbree and fonr 
retinal fibres immediately below tbe basement membrane (Fig. 20), 
a conditio-n also observable in Hotnai'us. In Palinurus^ according to 
ViALLANES (92, pag. 364), tbe fibres are arranged in mucb tbe same 
way as in Astacus except tbat tbere are five openings in tbe base- 
ment membrane instead of four, tbougb tbe latter number occasionally 
occurs. 
Tbe axis cylinder of eacb retinal fibre in Astacus passes as a 
transparent sbaft, tbe fibrillär axis, tbrougb tbe retinular cell to wbicb 
tbe fibre belongs, and disappears in tbe region of tbe rbabdome 
witbout extendiog beyond tbe distal end oftbat structure (cf. Figs. 21. 
20, 19, and 24, ax.u). Tbe position at wbicb tbe fibrillär axis 
disappears seems to me important as affording evidence in favor of 
the view tbat the rbabdome is tbe organ in wbicb tbe optic fibres 
terminate. In an earlier paper (Parker, 90, pag. 29), I described 
a similar axis in tbe retinular cells of Homarus^ and I subsequently 
identified like structures in a number of crustaceans (Parker, 91. 
pag. 116), results tbat bave since been confirmed by Yiallanes 
(92, pag. 362) in bis study of the eyes in Palinurus. 
In addition to the seven functional retinular cells just described, 
an eightb rudimentär y cell is present in Astacus^ as in many 
and perhaps all otber decapods. Tbe nucleus of tbis cell is bidden 
among the nuclei of tbe otber proximal retinular cells, and is seen 
with certainty only wben tbese nuclei are counted. Such an 
enumeration can be carried out in a series of consecutive sections 
tbrougb a single ommatidium as sbown in figures 8—13 (PI. 1). 
Here the nuclei are numbered in correspondence with the cells to 
wbicb tbey belong. Tbe most distal section (Fig. 8) contains parts 
of nuclei 1, 3, and 7, wbicb also appear in tbe next section (Fig. 9' 
togetber with a portion of nucleus 6. Nucleus 7 extends tbrougb 
to the tbird section (Fig. 10), in wbicb nucleus 6 is also represented 
and a part of nucleus 2. Tbe fourtb section (Fig. 11) contains tbe 
remainder of nucleus 2 and parts of nuclei 4 and 5, botb of wbicb 
reappear in the fiftb section (Fig. 12). Tbe sixtb section (Fig. 13) 
contains all seven cells and tbe eightb nucleus in its usual position 
on tbe dorsal face of cell 7; occasionally it occurs ventral to tbis 
cell but always associated with it. 
The body of tbe eightb cell seems reduced to a minimum, for 
its nucleus appears to be witbout surrounding protoplasm. In sec- 
