PARKER: RETINAL PIGMENT CELLS OF PALAMONETES. 289 
distal retinular cells. The body of the distal cell contracts in the light 
in an interval between 90 and 105 minutes, and expands in the dark 
in between 60 and 75 minutes, thus apparently accomplishing a change 
more rapidly in the dark than in the light. The expansion and con- 
traction are, however, not simple operations, but are complicated by the 
simultaneous production or absorption of the large proximal processes, 
and itis possible that the discrepancy just pointed out is to be accounted 
for by this complication. 
Before leaving this subject I wish to call attention to the compara- 
tive slowness with which all the photomechanical changes of the retina, 
but particularly those of the distal retinular cells, take place. Exner 
has shown that the amount of effective light that enters the eye is, in 
all probability, largely controlled by the action of the distal cells, and 
has therefore called them the iris pigment. The slowness with which 
they respond, however, shows clearly that in their action they have 
little resemblance to the iris of the vertebrate eye, and that their changes 
correspond only to the more general changes in the amount of light in 
their surroundings. The name iris pigment seems to me, therefore, 
somewhat misleading, and hence I prefer to retain the name of distal 
retinular cells, which indicates at once the present position and the 
probable origin of these cells, namely, from cells that once formed a 
part of the retinula itself (Parker, 95, p. 64). 
SYMPATHETIO PHOTOMECHANICAL JHANGES. 
To ascertain whether the retinas in the two eyes of Palwemonetes 
were sympathetic toward each other in the same sense that Engelmann 
believed the retinas in the eyes of vertebrates were, I carried out two 
sots of experiments, in both of which animals were so placed that one 
eye was in the dark while the other was exposed to the light. After a 
sufficient, period both eyes were prepared and examined. The two sets 
of experiments differed only in that I used different means to accom- 
plish the exposure. In one set I tied a living shrimp to the inside of a 
light-proof box, in which a small hole was made so as to allow one optic 
stalk of the animal to project into the lighted exterior. Jaro was taken 
that the small space between the optic stalk and the edge of the hole 
should be filled with an opaque material (a mixture of thick Janada 
balsam and lampblack). After several hours the animal was killed, and 
its eyes prepared. In the other set of experiments one optie stalk of a 
living animal was covered with a considerable quantity of the mixture 
VOL. XXX. — NO. 6. 2 
