No. 2.] EYES OF POLYCH^TOUS ANNELIDS. 173 



preparation is stained dark red, a red light. In a series of such 

 sections, the one immediately peripheral to Fig. 17 shows that 

 each round hole is continuous with the clear yellow central end 

 of a retinal cell (Fig. 18). These bright yellow areas are full of 

 yellow pigment spherules and surrounded by dense, dark blue 

 pigment. The blue pigment continues to surround the clear 

 axial parts of the cells till the level of the nuclei is reached 

 (Fig. 19), where it is very irregular and does not separate all the 

 cells. Final sections show only parallel cell processes, with 

 lines of blue pigment granules extending between and along 

 them. 



Though the dark pigment would appear to be between the 

 retinal cells, it is really in their superficial parts. For, in depig- 

 mented sections (Fig. 23) the area occupied by the blue pigment 

 is outlined by a clear non-granular substance, presenting no 

 nuclei nor indication of individuality other than that of the 

 retinal cells, for which this substance forms, as it were, a thick- 

 ened cell wall. 



Moreover, tangential sections partly depigmented (Fig. 15) 

 indicate that the pigment may be first removed from the thin 

 basal disk around the axial hole {a) while still remaining in the 

 very outermost boundaries of the cells {b). Comparing a and 

 b with Figs. 17 and 18, we conclude the blue pigment is most 

 refractory in the outermost surface of the cell, and may be 

 gradually removed from the axial region outwards, with no indi- 

 cation of break in continuity or sign of interpolation of special 

 small pigment cells between the large ones. When the depig- 

 mentation is complete, tangential sections (Fig. 16) present the 

 sharp polygonal outlines of the central ends of the retinal cells, 

 with no intervening spaces for nuclei or bodies of small pigment 

 cells. 



Macerations of the retina yield only one kind of cell (Fig. 9), 

 which is drawn out into a slender process at one end and con- 

 tinued as a clear refracting rod at the other. The dark pigment 

 adheres loosely to the cell except near the base of the rod, where 

 a dense mass persists ; this, however, may be finally removed, 

 revealing a slender stalk connecting the cell proper with the 

 rod. This stalk is the clear axial hole seen penetrating the 

 dense pigment zone in sections. The yellow pigment is found 

 as brilliant spherules in the axial parts of the cells, especially at 



