* 
316 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Cambarus is essentially the same. Extericrly the eye has the form 
of a stalk encased with tough cuticula and capped by a dome whose 
outer surface is the thin transparent continuation of the cuticula of 
the stalk. On the interior of the stalk, proceeding distad from its 
base, are the optic nerve, four large ganglia, and the retinal fibers 
which ascend from the ganglia to the retinal cells. Partitioning the 
stalk off from the dome is the basement-membrane or membrana 
fenestrata. Between the periphery of the dome and the basement- 
membrane lies the dioptric and receptive apparatus made up of many 
radial units or ommatidia. The components of a single ommatidium 
(cf. Plate 1, Fig. 1b) in centripetal order are these: — a corneal facet in 
the outer cuticula (ct); two subjacent corneal hypodermal cells (er); 
the cone (c) of four cells, differentiated into a distal, highly refractive 
part and a less refractive proximal portion; the spindle-like rhabdome 
(m), which abuts on the basement-membrane (bm); then, flanking 
the rhabdome are the seven proximal retinular cells (pp), while 
encompassing the outer segment of the cone are two distal retinular 
cells (dp), both sets of which, proximal and distal, contain brownish- 
black pigment-granules; finally, two sets of yellowish accessory or 
tapetal cells, a distal one lying between the distal retinular cells and 
the cone, and a proximal set (¢) situated, with respect to the rhabdome, 
outside of the proximal retinular cells. Of these ommatidial com- 
ponents the cone is regarded by Parker (’95) and Hesse (:01) as the 
dioptric apparatus, while the rhabdome is considered to be composed 
of differentiations of the proximal retinular cells and to be the receptive 
organ. It is only through this latter interpretation of the function 
of the rhabdome that the phenomenon of pigment-migration finds its 
significance. In Cambarus the réle which the distal retinular cells. 
play in the photomechanical changes of the eye is slight as compared 
with that of the proximal retinular cells. All my experiments dealt, 
therefore, with the influence of colored light upon the migration of 
pigment in these proximal cells alone. I shall often make use of the 
terms “retinal cells” and “retinal pigment” in referring to this set 
of cells. 
In order to establish a point of departure it was necessary to ac- 
quaint myself with the typical condition exhibited by an eye subjected 
to dark and by one subjected to light. Two standard eyes were 
prepared, one from an animal kept in absolute darkness for six hours. 
and killed in the dark, the other from an animal exposed for six hours 
to bright, diffuse daylight and killed in the light. Microscopic 
examination of the two showed diverse conditions. In the dark-eye 
(Plate 1, Fig. 2b) the retinal pigment lay almost entirely proximal to: 
