Langdon, Sense-organs of Nereis virens. 57 



Closely investing this central cylinder is a clear zone which is 

 continuous with the inner layer of the cuticula and which ap- 

 pears to consist of separate elements each of which belongs to 

 the peripheral end of a visual cell. These elements correspond to 

 the refractive bodies of the spiral organs. Between the bodies 

 of the visual cells and this clear zone is a layer of pigment. 



A comparison of the figures and descriptions of these vari- 

 ous eyes with the spiral organs of Nereis leads me to the con- 

 clusion that the latter must be simple epidermal eyes; that their 

 cells are retinal cells ; the ovoid and wedge-shaped parts of the 

 refractive bodies, rods and lens elements; and their central tube 

 merely an invagination of the cuticula. These spiral organs 

 need only a close packing of their lens elements and the addi- 

 tion of pigment to be strikingly like the eyes of several of these 

 Polychaetes. 



The absence of pigment might be taken as evidence against 

 the ocular character of these organs, or at least as evidence 

 that they were not at present functional. However, both Whit- 

 man ('89) and Nagel ('96) have arrived at the conclusion that 

 pigment is not a necessary part of a functional eye. 



The main difference in structure, besides the absence of 

 pigment, between these spiral organs and the ocular organs 

 previously described is the spiral arrangement of the peripheral 

 processes around the central tube — an arrangement which, so 

 far as I know, has never been described for any sense-organ. 

 This spiral seems at first to be the result of an actual whirling 

 around of the entire cells but the position of the cell-bodies 

 precludes this possibility. In reality I believe this spiral is due 

 simply to two processes which take place during development 

 — the increase in depth of the epidermis by a growth in length 

 of its elements and the sinking of the bodies of the cells be- 

 longing to a spiral organ below their former level. In the eyes 

 of Arthropods, Patten ('89) found that the clear refractive tips 

 of the visual cells were hexagonal and fitted closely together. 

 In Nereis the outer ends of the epidermal cells are hexagonal 

 and fit closely together. If a tubular invagination of such an 

 area of simple epidermal cells should take place, the invagin- 



