PAKKER: EETINAL PIGMENT CELLS OF PAL^MONETES. 281 



miuatcs, it was found throughout the bodies of the cells ; and, at thirty- 

 minutes, well marked concentrations had appeared about the rhabdome 

 and at the distal end of each cell. At forty-five minutes, these concen- 

 trations were somewhat more pronounced, but after that time no further 

 changes wei'e observable. 



The reverse change, which takes place in the dark, is accomplished in 

 the following manner. After tlie animal has been in the dark fifteen 

 minutes, the concentrations of pigment about tlie rhabdomes and at 

 the distal ends of the cells have almost disappeai-ed, though the bodies 

 of the cells still contain an almost uniform amount of pigment through- 

 out their whole length. After thirty minutes, much moi'e pigment is to 

 be found proximal to the basement membrane than distal to it, and after 

 forty-five minutes almost all the pigment is proximal in position. At the 

 end of an hour, the condition characteristic of darkness is fully realized. 



The changes just recorded occur entirely within the limits of each 

 proximal retinular cell. There is no reason for believing that the 

 changes are the results of a process of pigment production in one part 

 of the cell, and of pigment destruction in another. The observed facts, 

 on the contrary, suggest that the pigment granules of one region in the 

 cell are moved to another. The movement, however, is not accompanied 

 by any noticeable change in the position or even the form of the contain- 

 ing cell. The pigment granules seem to be carried up and down through 

 the cell, as though by a streaming of the cell protoplasm. A similar 

 stability of form, accompanied with an internal movement of pigment, 

 has been described by Ballowitz ('93, Taf. XXXVI. Fig. 12, and '93% 

 p. 629) in the pigment cells of the skin of fishes. 



Through the kindness of Professor F. H. Herrick, I have had the priv- 

 ilege of examining an interesting series of eyes taken from specimens of 

 Palsemonetes that had been kept living in a dark chamber thirty-eight 

 days. The pigment in the proximal retinular cells of such animals 

 showed the condition characteristic of the dark. In an animal that had 

 been kept in the dark for this period and then exposed to light for four 

 hours and three quarters, the pigment returned partially to the position 

 characteristic of the light. The greater part of it remained proximal to 

 the basement membrane, and from that which moved into the bodies of 

 the cells no marked concentrations were formed, either about the rhab- 

 domes or at the distal ends of the cells. Long confinement in the dark, 

 then, seems to interfere somewhat with the mechanism by which the 

 pigment of these cells is normally moved. 



The accessory pigment cells are located in the base of the retina, and 



