284 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY, 
In the interesting series of eyes of Palzemonetes loaned me by Professor 
Herrick, the accessory pigment cells of the eyes that had been kept in 
the dark thirty-eight days presented a condition normal for exposure to 
the dark. In those eyes that had afterwards been exposed to the light 
for four hours and three quarters, this pigment had apparently resumed 
the position normal for exposure to light. The mechanism by which the 
accessory pigment changes are brought about, unlike that for the proxi- 
mal retinular pigment changes, is therefore apparently not interfered 
with by prolonged retention in the dark. 
The distal retinular cells present photomechanical changes more com- 
plex than those in the two kinds of cells already considered. These 
changes have been described by Exner (’89 and '91), Szezawinska (’91), 
Herrick (91), and myself (Parker, 95). All investigators are agreed, 
I believe, in stating that in the dark these cells occupy a more distal 
position than in the light. Their probable influence on the amount of 
effective light that enters the retina led Exner (91, p. 63) to call them 
the iris pigment. In Palaemonetes, as I have already shown, there are 
two distal retinular cells for each ommatidium. 
In an animal that has been subjected to the full action of light, the 
distal retinular cells (Fig. 1, c/. dst.) are plump ovoid bodies in contact 
with the outer ends of the proximal retinular cells. The body of each 
distal cell has the length of about 30,4. From its outer end a single 
process usually extends to, or at least toward, the corneal hypodermis. 
The whole distal retinular cell, excepting its nucleus and sometimes a 
portion of its distal process, is filled with black pigment. The whitish 
pigment that often occurs on the outer surface of these cells repre- 
sents, as already mentioned, a distal process from the accessory pigment 
cells. 
In animals kept a sufficient time in the dark, the bodies of the distal 
retinular cells (Figs. 2 and 8, ed. dst.) are flattened, and applied to 
the sides of the cones. They measure about 70 u in length and pos- 
sess, in addition to their distal processes, shorter proximal ones, which 
extend backward to the outer ends of the proximal retinular cells. As 
before, the cytoplasm is largely filled with black pigment granules, 
which, however, are often more concentrated in the body of the cell 
than elsewhere. 
It must be obvious from this brief description that in considering the 
photomechanieal changes of the distal retinular cells two factors are to 
be kept distinct: first, the lengthening and the shortening of the cell 
body, and, secondly, the distal and the proximal migration of the cell as 
