No. 2.] COMPOUND EYES OF ANNELIDS. 279 



of the small pigment cells are not definitely arranged about the 

 refracting cells, but so that their cells are to be regarded as 

 filling in the spaces left amongst the large refracting cells. In 

 the inclusions of these refracting cells granules or spherical 

 droplets are often seen, as already noticed. 



In connection with these eyes of Potamilla, the common 

 pigmented, epithelial cells of the branchiae may be noticed. 

 As indicated in Fig. 1, the pigment of the large, transverse, 

 colored areas of each branchia is arranged in a reticulated 

 fashion, as seen magnified ; in fact, these reticulated pigment 

 lines correspond to the cell boundaries. That is, the ordinary 

 pigmented, epithelial cells often have their pigment granules 

 aggregated about their lateral surfaces as well as towards their 

 cuticular, outer ends, so that surface views frequently show the 

 axial part of each cell more nearly free from pigment, though 

 this is quite dense as seen from a side view. A surface view, 

 or an optical, transverse section, thus may present an appear- 

 ance similar to that shown in Fig. 9 for a tangential section of 

 the eye, only the pigment is much less, lighter, obviously con- 

 fined to the parts of the cells near their boundaries, and the 

 clearer, central parts are not as definite nor as refracting as in 

 the eye. 



II. Sabella. 



The reputed eyes upon the branchiae of several other Anne- 

 lids have also been examined, though less thoroughly than in 

 Potamilla. Of these, Sabella microphthalma, Verrill, is very 

 abundant at Wood's Holl, living in tubes, and having transverse 

 bands of pigment upon the branchiae, much as in Potamilla. 

 The so-called "eye spots" are not by any means confined to the 

 regions having pigmented filaments, the transverse bands, but 

 occur all along the outer side of each branchial stem in indefi- 

 nite numbers. These dark spots or blotches present no regu- 

 larity, but form series right and left of the median line of the 

 branchia, varying much in size, shape, position, and intensity of 

 color. Some spots may be as broad as 70 p., others merely 

 single pigment cells, with intermediate conditions. 



In a very young specimen, obtained in 1885 at Beaufort, 

 North Carolina, and about 5 mm. long, inclusive of branchiae, 



