The Retina and Optic Ganglia in Decapods, especially in Astaciis. 1 3 
nl.dst). Each cell completely covers two of tlie four sides of tlie 
cone from its distal to its proximal end, and continues beyond tliis 
proximally as a thick fibre, which is lost between the proximal 
retinular cells (Fig. 14., fbr.dst). The part of each cell applied to 
the cone always contains a rieh deposit of blackish pigment granules. 
SzczAWiNSKA (91, pag. 545) States, as mentioned above, that there 
are four such cells (cellules de l’enveloppe externe) for each omma- 
tidium, but, as CAEßii^RE (85, pag. 169) first showed, and my obser- 
vations confirm his, there are only two. 
The seven functional proximal retinular cells, enclosing 
the rhabdome, haye been seen in Astacus by Grenacher (79, pag. 125), 
Carriiire (85, pag. 169), and Szczawinska (91, pag. 546). Their 
swollen distal ends contain each a nucleus, and their proximal ones 
become fibrous, pierce the basement membrane, and extend as retinal 
fibres (Fig. 1 flr, r] to the first optic ganglion where they end in 
many fine branches (PI. 2 Fig. 46, fhr. r). 
ln transverse sections of the retinula, in which the mutual 
relations of these cells can be best studied, the rhabdome presents 
a squarish outline (PI. 1 Fig. 24), its four faces being distinguished 
from their positions as dorsal, ventral, anterior, and posterior. The 
first three mentioned are occupied each by two cells, which I have 
numbered 1 to 6 in the figures; the fourth, the posterior side, is 
covered by a single cell, numbered 7. The relation of the fibrous 
ends of the cone-cells and distal retinular cells to these seven cells 
is as follows (PI. 1 Fig. 14) : the fibres from the dorsal distal retinular 
cell and. from the dorsal cone-cell lie between cells 1 and 2, and 
the fibres from the corresponding ventral cells between cells 5 and 6 ; 
the fibre from the anterior cone-cell between cells 3 and 4, and that 
from the posterior cone-cell between either 7 and 1 or 7 and 6. 
The seven nerve fibres, representing the prolonged proximal 
ends of the seven retinular cells, pass through the basement mem- 
brane by means of four openings, each one of which, however, is 
in part occupied by fibres from an adjoining ommatidium. Num- 
bering the fibres in correspondence with the cells from which they 
arise, the groups that are formed in passing through the basement 
membrane are as follows: fibre 1 from a given ommatidium (Fig. 19) 
unites with fibres 4 and 5 from a neighboring one, and forms a 
single group; fibres 2 and 3 unite with the neighboring fibres 6 
and 7, and form a second group; fibres 4 and 5 unite wdth a neigh- 
boring fibre 1, and form a third group; and fibres 6 and 7 unite 
