Langdon, Sense-orga7is of Nereis virens. 1 5 



b. Diffuse sense-organs of the appendages. In the thickened 

 bases of the palps, of the cephalic cirri, and of the parapodia, 

 the diffuse sense-organs are exactly like those found in the 

 body-wall and the cell-bodies of these organs always lie in the 

 epidermis itself. 



In the slender distal portions of the cephalic and anal cirri, 

 in the tips of the palps and in the tentacles and parapodial 

 cirri, the diffuse sense-organs differ from those of the body itself 

 mainly in the following points : the inner cuticular cavity, ow- 

 ing to the greater thinness of the inner cuticular layer in these 

 appendages, is almost lacking, and the outer cavity is replaced 

 by an elevation ; the bodies of the cells of the sense-organs He 

 farther beneath the cuticula and those belonging to several 

 sense-organs are often massed together so that a given group 

 usually contains cells belonging to two or more organs. 



Each of the appendages mentioned in the last paragraph is covered by a 

 cuticula consisting of the same two layers found in the body cuticula, but in 

 this region the inner layer is only 2 or 3 /f thick and the outer layer only I or 

 2 l-i. Under the cuticula is a layer which is lacking in gland cells and is thus 

 composed of but one kmd of cell — the epidermal supporting cell. The bodies 

 of these cells are 16 fi in length ; they have their greatest width next to the 

 cuticula and taper gradually to a pointed base which in every case is prolonged 

 into one or more basal processes. These processes pass into the interior of the 

 appendage and there, together with the processes from a few stellate cells which 

 lie in this region, are loosely interwoven into fibrous tissue which fills the 

 greater part of the appendages. In the longitudinal axis of each appendage, 

 from its base almost to its apex, passes an axial nerve. 



Retzius ('92a and '92b) has considered that in such appendages, the bodies 

 of the epidermal cells form the epidermis itself, that the space between these 

 bodies and the axial nerve— that is, the space filled by the basal processes of the 

 epidermal cells — is beneath the epidermis; and that, therefore, the sensory 

 cells which lie in this region have sunken beneath the epidermis. Pruvot and 

 Racovitza ('95) and Racovitza ('96) have stated that in the Polychteta the 

 ♦' stylodes," i. e., the distal portions of the various sensory appendages — are 

 purely epidermal outgrowths. In my work on Nereis I have myself been led 

 to the same conclusion. In Nereis these appendages never contain an extension 

 of the body cavity. In them are found only structures which are found in the 

 epidermis of the body. In the latter, the bodies of the supporting cells of the 

 epidermis are the same in form and almost the same in size as those of the ap- 

 pendages, but one never thinks of locating the base of the epidermis of the 

 body at the base of the bodies of these cells. In this epidermis, the slender 

 basal processes of the epidermal supporting cells are almost as long as the basal 

 processes of similar cells in the appendages and extend centrally for a long dis- 



