Langdon, Sense-organs of Nereis virens. 6 1 



with the lens by a delicate strand. According to Hesse ('99) 

 the cells thus so connected with the lens are not visual cells but 

 " Sekretzellen " by which the lens has been formed. In sev- 

 eral eyes the striations always apparent in preserved lenses 

 passed obliquely to the longitudinal axis of the lens and their 

 arrangement strongly suggested the ovoid part of a spiral organ 

 — the part formed by the enlarged ends of the peripheral pro- 

 cesses, the refractive bodies, and the central tube — as seen in 

 poorly preserved material. The structure and the probable 

 function of these spiral organs and their position in line with 

 the eyes, suggests the possibility that they represent the more 

 primitive type from which the true eyes have developed. If 

 the true eyes were derived from the spiral organs, or both 

 are derived from the same more primitive organ, it is prob- 

 able that the lens of the true eye is, as Andrews rather 

 doubtfully suggests, actually composed of closely massed re- 

 fractive bodies, each of which is but the transformed peripheral 

 end of a retinal cell, and that the striations so often observed 

 in preserved eyes are caused by the shrinking apart of the lens 

 elements under the influence of reagents — a shrinking apart 

 directly comparable to the separation of the componet blocks 

 of the spiral band in a spiral organ. 



In Nereis diversicolor, Retzius ('95) was unable to find the 

 optic nerves. In my preparations of N. virens it has been very 

 easy to trace all four of these nerves from their respective eyes 

 into the lateral surface of the brain and thence to its posterior 

 part in which, according to Racovitza's ('96) investigations in 

 N. dumerili, lie the four optic ganglia. The nerve fibers from 

 the retinal cells are extremely delicate and in the methylene 

 blue preparations each is always broken up into a row of mi- 

 nute, disconnected granules. 



A number of very large unipolar ganglion cells are found 

 beside each of the anterior eyes. The greater number of these 

 cells are ventral to their eyes, but some are anterior and 

 some lateral to them. A few of the cells extend so far peri- 

 pherally among the epidermal cells as to almost touch the cuti- 

 cula. Several of them lie in the main dorsal branch of the circum.-. 



