316 bulletin: museum of comparative zoology. 



Cambarus is essentially the same. Exteriorly 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. lb) in centripetal order are these: — a corneal facet in 

 the outer cuticula (d) ; two subjacent corneal hypodermal cells (cr) ; 

 the cone (c) of four cells, differentiated into a distal, highly refractive 

 part and a less refractive proximal portion ; the spindle-like rhabdome 

 (?/?), which abuts on the basement-membrane (bm.); then, flanking 

 the rhabdome are the seven proximal retinular cells (pj^), 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 (t) 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 role which the distal retinular cells 

 play in the photomechanical changes of the eye is shght 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 



