PARKER: CHANGES IN RETINAL PIGMENT OF GAMMARUS. 147 



Summary. 



1. In Gammarus ornatus photomechanical changes in the retinal pig- 

 ment are not observable in the accessory pigment cells or in the distal 

 parts of the retinular cells, but are limited to the black pigment in the 

 middle and proximal portions of the retinular cells. 



2. In the light the middle portion of each retinular cell is well filled 

 with pigment, thus enclosing the rhabdome in a black sheath ; the 

 proximal portion contains scattered grains except near the nucleus, 

 where the pigment is more massed. 



3. In the dark the middle portion of each cell is almost free from 

 pigment, which now fills the proximal part. 



4. In the dark the removal of the black pigment from around the 

 rhabdome exposes the accessory pigment cells, which probably act as 

 reflecting organs, and in very dim light turn such rays as have escaped 

 laterally from the rhabdome back into that structure, thus aiding in 

 an effective stimulation of this organ. 



Cambridge, December 15, 1898. 



