The Ketina and Optìc Gatiglia in Decapods, especially in Astaciis. 25 
open the question as to whether his descriptions of tlie corresponding 
changes in other crustaceans are not faulty in this respect. With a 
view to partially answeriog this question, I studied the changes in 
the pigment of Palaemon^ one of the best instances given by Exner, 
and I can fully confirm his Statements regarding this crustacean; 
Palaemon certainly possesses a pigmented cylinder, which, under the 
influence of light or darkness moves proximalìy or distally over the 
united cone-cells. This shows that, though Exner's view may require 
some modification for certain crustaceans, as, for instance, Astacus^ 
it may stili hold good for others. Nor do the modifications required 
by Astacus necessitate any serious changes, for, if, in the presence 
of increased light, a pigmented cylinder is an optical advantage 
below the cones in obstructing oblique rays, pigmented processes in 
a similar position would ans wer the sarae purpose, though, of course, 
their action would probably be less complete. I regard the proximal 
processes of the distai retinular cells in Astacus as an apparatus of 
this kind, incomplete in one sense and yet efficient enough doubtless 
to meet the requirements of the animai. 
Concerning the accessory pigment cells with their whitish 
pigment, Szczawinska (91, pag. 552) states that under the influence of 
light they shorten and swell slightly. Exner (91, pag. 105), however, 
was unable to identify with certainty any movement in these cells 
in Palaemon^ and regards the changes that they appeared to undergo 
as due to the fact that in different conditions of Illumination they 
were differently covered by the blackish pigment of the proximal 
retinular cells. My own observations on Astacus support Exner's 
conclusion; in eyes preserved in darkness, as well as in those pre- 
pared in light, the accessory pigment cells extend from the middle 
of the rhabdome to the distai face of the basement membrane, and 
in these two conditions showed, as far as I could observe, no differ- 
ence in form. I therefore conclude that they are not affected by 
light or darkness. 
In a retina exposed to light, the black pigment in the proximal 
retinular cells lies between the accessory pigment cells and the 
rhabdome; but, in one from which the light is excluded, the black 
pigment migrates to a region below the basement membrane, and 
there is no opaque substance intervening between the accessory cells 
and the rhabdome. If, under such circumstances, a small amount 
of light were to enter the retina and pass to the proximal end of 
the rhabdome without being absorbed, it would probably be reflected 
