282 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



send a few processes distad to the outer surfaces of the distal retinular 

 cells, and many proximad through the apertures in the basement mem- 

 brane to the distal surface of the first optic ganglion. The pigment 

 with which these cells seem to be almost entirely filled is yellowish by 

 transmitted light, and white by reflected light. It is especially remark- 

 able for its powers of reflecting light, and this quality led Exner ('91, 

 p. 97) to designate the layer formed from it by the name of the tape- 

 turn. Whether this pigment is influenced by the presence or absence 

 of light is a matter of some uncertainty. Szczawinska ('91, p. 552) 

 states that in Astacus, under the influence of light, the cells containing 

 it enlarge slightly. Exner ('91, p. 105), though at first inclined to re- 

 gard the accessory cells as influenced by the light, was finally led to 

 abandon this view, and to explain their two apparent conditions by the 

 greater or less degree with which they were covered by the migi'ating 

 pigment of the proximal retinular cells. In a preparation from an eye 

 kept in the dark, the retinular pigment, as already mentioned, is entirely 

 below the basement membrane, and the accessory pigment is almost en- 

 tirely exposed, and consequently conspicuous. In the light it is some- 

 what covered by the black pigment, which under these circumstances 

 fills the bodies of the proximal retinular cells, and it thus becomes less 

 noticeable than before. My own studies on the retina of Astacus (Pai-- 

 ker '95, p. 25) led me to agree with Exner that the accessory pigment 

 showed only an apparent change. If, however, any change did occur, it 

 was certainly not an increase in the size (conspicuousness T) of the acces- 

 soiy cells under the action of light, as maintained by Szczawinska, but 

 rather the reverse. 



Although in respect to Astacus I am still in doubt as to whether or not 

 the accessory cells show any photomechanical changes, I have not the 

 least hesitancy in stating that in Palaemonetes such a change does occur. 

 The principal difiiculty in demonstrating this change comes from the dis- 

 turbing influence produced by the migration of the pigment in the proxi- 

 mal retinular cells. This difficulty, however, can be overcome by the 

 employment of a depigmenting reagent that will remove the retinular 

 pigment without afl'ecting the accessory pigment. Such a reagent is 

 the depigmenting fluid recommended by Grenacher ('86, p. 214). In 

 preparations representing the dai'k and the light condition, and depig- 

 mented by this means, the diff'erences in the distribution of the pigment 

 in the accessory pigment cells is so striking that no one would question 

 for a moment the photomechanical activities of these cells. In the light 

 (Fig. 1) the accessory pigment forms two concentrations, one in the base of 



