22 
G. H. Parker 
penetrates and fixes the tissue almost at once. In this respeét it is, 
perhaps, a safer reagent tlian fluids that enter the tissues more 
slowly and thus allow an opportunity for slight changes in the po- 
sition of the pigment. However, on coniparing eyes hardened in 
alcohol or corrosive sublimate with those prepared in hot water, I 
never discovered any important differences so far as the position of 
the pigment was concerned. 
As may be gathered from the anatomical description already 
given, the pigment in the retina of Astacus is of two kinds, fine 
blackish grains, and flakes of an irregularly cry stalline substance, 
yellowish by transmitted light but whitish in reflected light. These 
two substances are contained in different cells; the flaky material 
fills the accessory pigment cells and constitutes what Exnek calls 
the tapetum, while the blackish grains are found in the distal and 
proximal retinnlar cells, corresponding to what Exner has termed 
the iris and the retinal pigment respectively. 
The pigment in the proximal retinular cells is easily affected 
by light. In diffused daylight, it is present thronghout the retinular 
cell from its distal nucleated end to a point in its fibrous Prolon- 
gation some distance below the basement membrane, the concentration 
of the pigment, however, being around the distal end of the rhab- 
dome (PI. 1 Fig. 1); in other words, by means of this pigment the 
whole rhabdome is protected from the approach of light except at 
its distal end, where light can gain access to it through the small 
aperture by which the four cone-cells penetrate the distal end of 
the retinula, as previously described. Under similar circumstances, 
essentially the same arrangement of pigment was observed by 
SzczAWiNSKA (91, pag. 552) and by Exner (91, pag. 109). 
In eyes that had been prepared in perfect darkness, the 
pigment of the proximal retinular cells was found to have migrated 
completely into the retinal fibres below the basement membrane 
(PI. 1 Fig. 2) ; in other words, in complete darkness, no part of the 
rhabdome remains covered by black pigment; and, except for the 
protection that its proximal end receives from the accessory pigment 
cells, it is accessible to the light from all sides. The extreme 
position of the pigment by which this condition is produced has 
been observed by Exner (91, pag. 109). According to Szczawinska 
(91, pag. 552) , however, the pigment does not retreat proximally 
further than the middle of the rhabdome. I have observed this 
condition in crayfishes that have been kept in the dark and whose 
