PARKER: RETINAL PIGMENT CELLS OF PALAIMONETES. 279 
none proved so satisfactory as water at about 80° C. The periods of 
exposure to dark in the first set of experiments were as follows: 1 min., 
5 min., 10 min., 15 min., 30 min., 45 min., 60 min., and then at intervals 
of an hour up to 8 hours. It was found subsequently that the experi- 
ments need not have extended over a maximum period of more than 
two hours, and that intervals of about fifteen minutes were all that 
were needed to observe the steps of the change. From each lot of ani- 
mals prepared in this way, the optic stalks were out into sections for 
examination under the microscope. In a similar way, the eyes of ani- 
mals that had been kept some four hours in the dark were exposed to 
the light for given intervals, killed, cut, and examined. In cases where 
it was necessary to make very accurate comparisons, the eyes of the 
same animal were used for the two conditions; thus, after keeping 
the animal a given time in the light, one optic stalk was removed, and the 
animal kept in the dark. At the expiration of the second interval, the 
second optie stalk was removed and prepared. To guard against indi- 
vidual varietions, in every experiment the eyes of at least, three animals 
were examined. 
The only general changes shown by retinas subjected to light or dark 
were changes in the arrangement of the pigment. In other respects 
they were not noticeably altered. Thus, no change in thickness was 
observable; in one case, a left retina that had been kept in the dark 
measured in its middle region from the corneal cuticula to the basement 
membrane 263 % while the right retina from the same animal exposed 
to light measured 270 u. In à second ease, a dark left retina measured 
240 % tho light right one measuring 233 u. The cones likewise showed 
ao significant differences. By analogy with the perceptive elements in 
the vertebrate eye, one might have expected the rhabdomes, the termi- 
nal nervous organs of the crustacean eye, to shorten in the light and 
lengthen in-the dark. I was unable to obtain evidence of such a change 
in Palemonetes, and yet the conditions for the exact measurement of 
the rhabdomes are so unfavorable in this animal that I am by no means 
certain that these changes may not occur If, however, they do take 
place, they must be relatively small. The observable changes induced 
in the retina by the absence or presence of light affect the three kinds 
of pigment cells, — the proximal retinular cells, the accessory cells, and 
the distal retinular cells. These will be considered in the order given. 
The pigment in the proximal retinular cells forms at the base of the 
retina a band, called by Exner (91, p. 62) the retinal pigment. The 
photomechanical changes that this pigment undergoes have already been 
