PARKER: RETINAL PIGMENT CELLS OF PAL^EMONETES. 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 it is 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 the-sg cells, namely, from cells that once fonned a 

 part of the retinula itself (Parker, '95, p. 64). 



Sympathetic Photomechanical Changes. 



To ascertain whether the retinas in the two eyes of Palsemonetes 

 were sympathetic toward each other in the same sense that Engelmann 

 believed the retinas in the eyes of vertebrates were, I carried out two 

 sets 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- 

 pHsh 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 liglited exterior. Care 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 Canada 

 balsam and lampblack). After several hours the animal was killed, and 

 its eyes prepared. In th^ other set of experiments one optic stalk of a 

 living animal was covered with a considerable quantity of the mixture 



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