Langdon, Sense-organs of Nereis virens. 17 



2, and Plate II, Fig, 38). Such a group may be considered as 

 made up of as many organs as there are cuticular areas to which 

 its peripheral processes pass. In this case it may at first seem 

 difficult to distinguish a grouping of cells into definite organs. 

 But one may suppose that originally the sense-organs lay nearer 

 the cuticula — as is the case in the body wall, that in the base of 

 the epidermis the central fibers from several organs passed into 

 a small nerve, and that several of these smaller nerves joined to 

 form a larger one. If now the bodies of the cells of these or- 

 gans sink centrally from their original position, their course 

 would naturally be along the course of their central processes 

 and they would come to lie somewhere in the original course of 

 these processes. In those cases in which the central processes 

 from two or more organs soon joined to form a common nerve, 

 the bodies of the cells of these organs would come to lie side 

 by side at this point of junction or even along the former course 

 of this common nerve itself; and the original course of the 

 outer portion of the central processes from these organs would 

 be shown by the final position of the inner portions of their 

 peripheral processes. In the cepJialic cirri, therefoir, I consider 

 that one of the modified areas through zuhich a group of sensoiy 

 hairs pass to the exterior, the small kindle of peripheral processes 

 going to this area and beating these sensory hairs, and the cells giv- 

 ing ofigin to these peripJiei'al piv cesses, whetJier associated with 

 cells belonging to other organs or not, constitute a sense-organ. 

 Such an organ would be directly comparable with the diffuse 

 sense-organs found over the general body of Nereis. 



In these cephalic cirri, a group of sensory cells varies in 

 length from 25 to 80 /<, in width from 12 to 16//, and contains 

 from 7 to 16 cells. The cells composing a single group lie at 

 different depths beneath the cuticula (Plate I, Fig. i). Occa- 

 sionally the body of one cell is separated from the others, but 

 usually the body of one fits between the tapering ends of oth- 

 ers. There is a greater difference in the form of these cells 

 than is found in the cells of the diffuse sense-organs of the body 

 epidermis, seemingly because in the latter region the cells are 

 more pressed upon by adjoining tissue. In the general epider- 



