The Retina and Optic Ganglia in Decapods, especially in Astacus. 21 
is, as a rule, a rich deposit of fine blackish pigment granules. The 
whole element thus briefly outlined represents a single celi, or, 
adopting the term suggested by Waldeyer (91, pag. 52) for such 
nervoiis Clements, a neuron. 
e. Migration of Retinal Pigment. 
The changes in position produced in the pigment of crustacean 
eyes by the action of ligbt and darkness bave been described, so 
far as I am aware, by only two investigators, Szczawinska and 
ExNER. In the account by Szczawinska (91), these changes are 
dealt with chiefly from an anatomical standpoint, while Exner (91) 
has regarded them rather in the light of their physiological signi- 
ficance and has brought them into intimate relations with his theory 
of vision in compound eyes. Both Exner and Szczawinska bave 
studied the crayfish, and, though their results agree in the main, 
these stili present differences important enough to cali for rein- 
vestigation. 
The method that I fìnally adopted for preparing the eyes was 
somewhat more complicated than that formerly used, but the greater 
exactness of the results yielded by it seems to me to justify its 
employment. The extreme conditions to which I subjected the cray- 
fìshes were, on the one band, darkness as complete as could be 
obtained in a closed dark-room and, on the other band, daylight 
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 which the animai lived, and 
I did not resort to the use of excessively bright light, such as direct 
sunlight, etc., as used by Szczawinska and which, as she remarks, 
often kills the animals. To ward against possible individuai varia- 
tions in the amount of pigment in the eyes, I prepared both eyes 
in each animai, one after subjecting the animai to the light for four 
hours and the other after it had been in darkness for the same 
period. xis the removal of one eye does not interfere with the 
normal action of the other, so far as I could observe, I see no ob- 
jection to this method of procedure, which certainly has the advan- 
tage of allowing a closer comparison of the condition in the two 
eyes. The eyes subjected to darkness were prepared without 
exposure to light, usually by hardeniug the tissues of the animai 
as a whole. This I did by means of water at about 90° C; the beat 
