PARKER: RETINAL PIGMENT CELLS OF PALAIMONETES. 277 
optic ganglion by the retinal nerve fibres. Nerve fibres connect the 
first optic ganglion with the second, the second with the third, and the 
third with thé fourth. From the fourth optic ganglion, which is sit- 
uated near the proximal end of the stalk, the optic nerve extends to the 
brain. The finer structure of the optic ganglia in Palaamonetes is in all 
probability essentially the same as in Astacus, where, as I have already 
shown (Parker, '95, Taf. 3, Fig. 59), each optic ganglion represents a 
region of interruption for the great majority of the nerve fibres that 
intervene between the retina and the brain. 
The retina in Palemonetes is composed of ommatidia, the structure of 
which has already been described at length (Parker, '91, p. 108, Pl. IX.). 
For convenience I add a brief summary of this description. Each omma- 
tidium is composed of five kinds of cells. Immediately under the cor- 
neal facet (Fig. 1, ern.) are two corneal hypodermal cells (nl. ern.). The 
distal portion of tho axis of the ommatidium is occupied by the cone (con.), 
which, as seen in transverse sections (Fig. 3, cl. con.), is composed of four 
parts. Each part contains near its distal end a nucleus (Fig. 1, nl. con.) 
and represents a cell The four cone cells are closely applied to one an- 
other in the region of the cone proper (Fig. 3). Proximally they taper 
off as thick, more or less independent fibres. (Compare Figs. 1, 4, and 5.) 
These fibres separate and apparently terminate near the distal end of 
the rhabdome (Fig. I, r4b.). I have been unable to trace them further, 
though I suspected that they might end, as in Homarus (Parker, '90, 
p. 14), on the basement membrane. The distal retinular cells either 
apply themselves to the lower portion of the sides of the cone (Fig. 2, 
cl. dst.), in which case they are so closely packed that their outlines 
cannot be distinguished (Fig. 3, el. dst.), or they oceupy a more prox- 
imal position (Fig. 1, el. dst.), forming a ring around the attenuated ends 
of the cone cells (Fig. 5). There is, of course, one ring for each om- 
matidium. Each ring contains six distal retinular cells, but these rings 
are so constituted that each cell is at the same time a member of three 
rings; hence there are in reality only twice as many distal retinular 
cells as there are ommatidia. The proximal portion of the axis of the 
ommatidium is occupied by the rhabdome (Fig. 1, /.), which is sur- 
rounded by seven functional proximal retinular cells (Wig. 6, el. px.), 
in addition to which an eighth rudimentary one is present (Parker, ’91, 
p. 111). Each funotional cell ends distally in a somewbat swollen knob 
containing its nucleus (Fig. 1, nd. px.). From this swollen end the 
cell extends proximally over the rhabdome, beyond whieh it becomes 
slightly attenuated, and, as a retinal nerve fibre (Figs. 1 and 7, for. r.), 
