No. 2.] EYES OF POLYCH^TOUS ANNELIDS. 213 



With a greater elongation of the cells (4), a pigmented retinal 

 cup was formed containing the lens mass, now more efficient 

 and gradually divided, in some cases, into a lens proper near 

 the cuticle and an intervening vitreous body next the rods. 

 Such eyes remain as the perfect adult structures in many cases 

 (Figs. 3, 8, 30, 31, 32, 33, 34, 46, 49, 52). 



Any component cell will have the parts represented in Dia- 

 gram I, a nerve fibre passing to the brain, a pigmented part, a 

 clear rod, and a part of the common refracting mass in contact 

 with the cuticle. 



Finally, in some perfect eyes, as seems the case in the Alcio- 

 pidae, and in some degenerating eyes, the sinking in from the 

 surface may extend so far that the eye is entirely removed from 

 the cuticle (5), and may then, apparently, become closed in to 

 form a complete vesicle with no connection with the epidermis. 

 This last phase is, however, an unusual one, and may possibly 

 not really exist in any described eye. 



In all that precedes, reference has been had to the eyes upon 

 the cephalic lobe proper, near the brain, and especially well devel- 

 oped in errant Polychaetae. The peculiar branchial eyes of some 

 sedentary groups may, however, be brought under the same gen- 

 eral conceptions, at least as to their origin. 



As illustrated in a former article in Vol. V. of this Journal, 

 the numerous epidermal eyes upon the branchial processes of 

 the heads of some tubicolous Polychaetae differ from the eyes 

 described in the present paper, in that each cell has its refracting 

 body isolated from that of the others. There is thus no multi- 

 cellular lens, but a compound eye or aggregate of separate cells 

 each an eye in itself. 



These may be regarded as originating in a simple epithelium 

 represented in i, Diagram 2. Subsequently, however, the clear 

 part of the cell became specialized only in the axial part of the 

 cuticular end, not throughout it as in the stage 2. In this axial 

 part, also, the lens-like body was found still in contact with the 

 cuticle as in 3. 



The elongation of the cells, each having a refracting body 

 within its cuticular end, did not result in the formation of a large 

 lens mass and, moreover, there was no sinking, but rather an 

 elevation of the entire region. This elevation is, perhaps, con- 

 nected with the necessities of a functional compound eye, but 



