No. 2.] COMPOUND EYES OF ANNELIDS. 273 



pigment, and a less evident process from the apical or cen- 

 tral end. 



With little pressure and percussion much of the pigment may 

 be removed from the outside of the refracting cell, and then the 

 axial part is seen to contain a nucleus about the middle of its 

 length, and to present in the basal, peripheral part a peculiar 

 refracting body staining dark blue in the methyl green. 



The most intelligible views of the axial part of these refract- 

 ing cells were obtained after maceration in sea-water containing 

 a small amount of sulphuric acid, — a method suggested to me by 

 Professor Patten. The pigment, now of a bright red color, is 

 no longer firmly attached to the cell, but readily falls off, reveal- 

 ing the clear, non-pigmental, axial parts. In the central part of 

 the cell the conspicuous nucleus (Fig. 4) stains dark red with 

 Mayer's acid carmine, or lavender color with Czokor's cochineal, 

 or faintly bluish in methyl green, just as do the nuclei of the 

 ordinary epithelial cells of the branchiae. The large, peripheral 

 end of the cell is sharply defined, refracting, and very noticeable, 

 as containing a clear, highly refracting, vase-shaped body, nearly 

 filling this end of the cell. This body or inclusion stains very 

 dark blue in methyl green, and dull red, less intense than the 

 nucleus, in Mayer's acid carmine. The peripheral end of this 

 inclusion is a cylindrical projection passing to the centre of the 

 ♦basal or peripheral end of the cell, while the central end is 

 abruptly truncated, and forms the base of the vase itself, and 

 coincides with a sudden transition from this peripheral part of 

 the cell to that next to be described. The axis of the cell in 

 this last region, between the above inclusion and the nucleus, 

 presents a series of rounded disks, or parts of the cell, differing 

 in refractive effect from the rest, and slightly differentiated when 

 stained in methyl green. Finally, the apical part of the cell 

 beyond the nucleus is homogeneous, and passes over into a 

 slender process usually broken off in these preparations. 



This refracting cell thus has within its protoplasm a nucleus, 

 a remarkable, very highly refracting body of peculiar shape, and 

 between these an axial series of faintly differentiated parts. 



The whole is covered by pigment, and has often adhering to 

 it smaller pigmented cells with a single nucleus, but with neither 

 of the above unusual parts characteristic of the large refract- 

 ing cells. 



