146 bulletin: museum of comparative zoology. 



the cone, and has been fully described by him. In accordance with this 

 relation, then, the rhabdome of each omniatidium would receive light 

 from an external region corresponding to the outward projection of the 

 axis of the cone distal to it. 



As the pigment which surrounds the cone is merely concerned with 

 the absorption of the lateral rays, and as these rays would be equally 

 disturbing whether the animal were in bright light or dim light, it fol- 

 lows that no photomechanical changes in correspondence with changes in 

 the intensity of the light should be expected in this pigment, and as a 

 matter of fact in G. ornatus no such changes have been observed. 



The axial light, which according to the foregoing account finds its way 

 into the rhabdome, must have a certain degree of intensity in order to 

 stimulate that organ. Ordiuary daylight is presumably more than suffi- 

 cient to call forth this stimulation, and such superfluous light as may 

 pass to the edges of the rhabdome or through it is probably absorbed by 

 the black pigment that in bright light (Figs. 1, 2) surrounds that body. 

 In dim light, however, there must be times when the light which en- 

 ters the rhabdome is scarcely intense enough to stimulate that organ. 

 Under such circumstances the more oblique rays, which ordinarily would 

 be absorbed by the black pigment on the sides of the rhabdome, would 

 materially aid in stimulating it if they were turned back into the rhab- 

 dome. That these rays are probably thus turned back is shown by the 

 fact that in dim light the black pigment is removed from the rhabdome 

 and the surrounding whitish reflecting pigment of the accessory pigment 

 cells is exposed (compare Figs. 2, 4). 



Changes exactly comparable with these have been described in the 

 proximal retinular cells of the higher mistaceans. The changes in the 

 distal retinular cells, the iris pigment of Exner, which are connected 

 with the formation of superposition images, find no representatives in 

 the eyes of Gammarus ornatus. As this eye is in many respects primi- 

 tive, it is likely that the ancestral crustacean ommatidium possessed a 

 catoptric cone and a retinula provided with a reflecting apparatus for 

 use in dim light. In the differentiation of the higher type of crustacean 

 ommatidium the reflecting mechanism was retained, but the catoptric 

 cone gave way to a second type of cone provided with special pigment 

 cells whose movements were associated with the superposition images 

 formed by this type of cone. 



