No. 2.] 



COMPOUND EYES OF ANNELIDS. 



275 



layer, which, in fact, might be regarded as outside the cell, from 

 the defmiteness of the apparent cell outlines when deprived of 

 pigment. 



Such macerated eyes before disassociation show the clear, 

 refracting cells, radiating out from the central region of the eye 

 towards the cuticle that covers the entire organ, 

 the small process of each vase-shaped inclusion 

 coming, apparently, into contact with that cuticle. 

 These cells are now seen to be imbedded irregu- 

 larly in a mass of pigment cells, such as seen in 

 Fig. 3, each refracting cell being one of the clear, 

 red areas seen in Fig. 1, where we are to suppose 

 the red light transmitted through the clear, axial 

 parts of the cell comes to the observer as differ- 

 ent from that reflected from the more peripheral, 

 similarly pigmented parts. On the addition of 

 strong potash the pigment is removed, and the 

 twenty to forty refracting cells seen in optical 

 section are distinctly recognizable as the same 

 made out in maceration, each being now some- 

 what shrunken away from its fellows, and from 

 the smaller cells; pigment cells. 



Branchiae depigmented and macerated in ten per cent nitric 

 acid show that the eye is the end of a gradual thickening of the 

 epidermis, an elongated ridge rising gradually, to end suddenly 

 at the distal edge of the eye. Hence longitudinal optical sec- 

 tions show the epidermal cells becoming gradually longer as the 

 eye is approached from the proximal side, longest in the eye 

 itself, and finally suddenly returning at the distal side of the eye 

 to the usual cuboidal epidermis of the rest of the branchial sur- 

 face. Transverse sections, on the other hand, show this ridge 

 passing off gradually and equally on each side into the common 

 cuboidal epithelium. The eye is thus, as it were, at the tip of a 

 rudimentary external process, a mere ridge, however. The struc- 

 ture of the eye, as now seen, is evidently merely a modification 

 of the general epithelium of the branchia, an elevation in which 

 many cells have nuclei near the outer, peripheral ends ; others, 

 the large, refracting cells, their nuclei much nearer to the cen- 

 tral part of the eye, remote from the cuticle. The arrangement 

 of the two sets of cells is not definite ; the pigment cells sur- 



