The Retina and Optic Ganglia in Decapods, especially in Astacus. 23 
eyes have been preserved immediately on exposing the animals to 
the light. As this seems to have been Szczawinska’s nsual method 
of procedure, it is easy to understand why sbe never observed the 
pigment in its more extreme position as seen by Exner and myself. 
In all cases, however, in which the eyes were prepared in darkness 
after having been kept there some hours, the absence of retinal 
pigment on the distal side of the basement membrane was easily 
demonstrated. 
In dim light, as might be expected, this pigment occupies a 
Position intermediate between the two extremes just described and 
covers the proximal half or two-thirds of the rhabdome. From its 
action it seems to be a means of Controlling the access of light to 
the rhabdome; when much light enters the eye, it covers the rhab- 
dome all but the distal end; on decreasing the amount of light the 
exposed surface of the rhabdome is increased, tili finally in perfect 
darkness the whole rhabdome is uncovered. 
Since the changes shown by the pigment in the distal reti- 
nular cells are not so easily observed as those in the cells last 
described, it is not surprising that the accounts of Szczawinska and 
Exner do not agree in this instance as well as in the former. Each 
distal retinular cell, it will be remembered, consists of two parts: a 
flat portion, which may be called the body of the cell and which 
covers two adjacent faces of the cone, and a process, which extends 
from the proximal edge of the body to the retinula. In regard to 
the distribution of pigment in these parts, considerable individual 
Variation occurs, and I begin by describing a case that may be said 
to represent one extreme of these differences. 
In a left eye prepared in darkness, pigment was observed only 
in the bodies of the distal retinular cells (PI. 1 Fig. 2), their proxi- 
mal processes containing none of this substance; in other words, 
the pigment was lodged entirely between the cones as observed by 
Exner (91, pag. 73) and pj Szczawinska (91, PI. 17 Fig. 1). In 
the other eye from the same animal prepared in da y light, the pig- 
ment had evidently migrated proximally (PI. 1 Fig. 1), not, however, 
as a pigmented cylinder sliding over the cone as described by Exner, 
but as pigmented processes, one for each cell as stated by Szcza- 
winska (91, pag. 545). When viewed from the side, however, these 
processes seem like the edges of a cylinder cut longitudinally, an 
appearance that probably misled Exner. In serial transverse sections, 
however, the fibrous form of the processes can be demonstrated 
