The Retina and Optic Ganglia in Decapods, especially in Astacus. 21 
is, as a rule, a rieh deposit of fine blackish pigment granules. The 
whole element thus briefly outlined represents a single cell, or, 
adopting the term suggested by Waldeyer (91, pag. 52) for such 
nervous elements, a neuron. 
e. Migration of Retinal Pigment. 
The changes in position produced in the pigment of crustacean 
eyes by the action of light and darkness have been described, so 
far as I am aware, by only two investigators, Szczawinska and 
Exner. In the account by Szczawinska (91), tbese changes are 
dealt witb ebiefly from an anatomical standpoint, wbile Exner (91) 
bas regarded tbem rather in the light of tbeir pbysiological signi- 
ficance and bas brougbt tbem into intimate relations witb bis tbeory 
of Vision in compound eyes. Botb Exner and Szczawinska have 
studied the crayfish, and, thougb tbeir results agree in the main, 
tbese still present differences important enougb to call for rein- 
vestigation. 
The method tbat I finally adopted for preparing the eyes was 
somewbat more complicated than tbat formerly used, but the greater 
exaetness of the results yielded by it seems to me to justify its 
employment. The extreme conditions to wbicb I subjected the cray- 
fisbes were, on the one band, darkness as complete as could be 
obtained in a closed dark-room and, on the otber band, dayligbt 
bright but diffused. As my object was to study the normal action 
of the eyes, I tried to reproduce, so far as the light was concerned, 
the extreme natural conditions under wbicb the animal lived, and 
I did not resort to the use of excessively bright light, such as direct 
sunligbt, etc., as used by Szczawinska and wbicb, as she remarks, 
often kills the animals. To ward against possible individual varia- 
tions in the amount of pigment in the eyes, I prepared botb eyes 
in eacb animal, one after subjecting the animal to the light for four 
bours and the otber after it had been in darkness for the same 
period. As the removal of one eye does not interfere witb the 
normal action of the otber, so far as I could observe, I see no ob- 
jection to tbis method of procedure, wbicb certainly bas the advan- 
tage of allowing a closer comparison of the condition in the two 
eyes. The eyes subjected to darkness were prepared witbout 
exposure to light, usually by bardening the tissues of the animal 
as a whole. Tbis I did by means of water at about 90° C.; the heat 
