282 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
send a few processes distad to the outer surfaces of the dista] retinular 
cells, and many proximad through the apertures in the basement, mem- 
brane to the distal surface of the first optic ganglion. The pigment 
with which these cells seem to be almost entirely filled is yellowish by 
transmitted light, and white by reflected light. It is especially remark- 
able for its powers of reflecting light, and this quality led Exner (91, 
p. 97) to designate the layer formed from it by the name of the tape- 
tum. Whether this pigment is influenced by.the presence or absence 
of light is a matter of some uncertainty. Szezawinska (91, p. 552) 
states that in Astacus, under the influence of light, the cells containing 
it enlarge slightly. Exner (91, p. 105), though at first inclined to re- 
gard the accessory cells as influenced by the light, was finally led to 
abandon this view, and to explain their two apparent conditions by the 
greater or less degree with which they were covered by the migrating 
pigment of the proximal retinular cells. Ina preparation from an eye 
kept in the dark, the retinular pigment, as already mentioned, is entirely 
below the basement membrane, and the accessory pigment is almost en- 
tirely exposed, and consequently conspicuous. In the light it is some- 
what covered by the black pigment, which under these circumstances 
fills the bodies of the proximal retinular cells, and it thus becomes less 
noticeable than before. My own studies on the retina of Astacus (Par- 
ker '95, p. 25) led me to agree with Exner that the accessory pigment 
showed only an apparent change. If, however, any change did occur, it 
was certainly not an increase in the size (conspicuousness ?) of the acces- 
sory cells under the action of light, as maintained by Szezawinska, but 
rather the reverse. 
Although in respect to Astacus I am still in doubt as to whether or not 
the accessory cells show any photomechanical changes, I have not the 
least hesitancy in stating that in Palemonetes such a change does occur. 
The principal difficulty in demonstrating this change comes from the dis- 
turbing influence produced by the migration of the pigment in the proxi- 
mal retinular cells. This difficulty, however, can be overcome by the 
employment of a depigmenting reagent that will remove the retinular 
pigment without affecting the accessory pigment, Such a reagent is 
the depigmenting fluid recommended by Grenacher (’86, p. 214). In 
preparations representing the dark and the light condition, and depig- 
mented by this means, the differences in the distribution of the pigment 
in the accessory pigment cells is so striking that no one would question 
for a moment the photomechanical activities of these cells. In the light 
(Fig. 1) the accessory pigment forms two concentrations, one in the base of 
