284 bulletin: museum of comparative zoology. 



lu the interestiug series of eyes of Palsemonetes loaned me by Professor 

 Herrick, the accessory pigmeut 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. 



Tlie 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), Szczawinska ('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 Palsemonetes, 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, cl. 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 /x. 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, cl. dst.) are flattened, and applied to 

 tlie sides of the cones. They measure about 70 ^ 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 

 photomechanical 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 



