i6 Journal of Comparative Neurology. 



tance before forming a basement membrane. This basement membrane lies 

 between the epidermis itself and the layer of circular muscles just beneath it. 

 In the distal parts of the cephalic, anal, and parapodal cirri, and in the ten- 

 tacles there are no muscles whatever ; those found in the retractile tips of the 

 palps do not form a circular muscle layer, but are the ends of muscles which 

 enter these appendages exactly as similar muscles enter the epidermis of the 

 body itself. These appendages must be, therefore, purely epidermal outgrowths — 

 strictly homologous with the epidermis itself^and the base of the epidermis of any 

 one of these structures can only be found at the base of the structure itself. The 

 axial nerves of each appendage would then be but one of the nerves which lie 

 in the base of the epidermis and receives the central processes of several 

 sense-organs. 



It will be observed that the gills and the thickened bases of the palps and 

 of the anal and cephalic cirri are not included in this discussion. All of these 

 structures contain an extension of the body cavity and the base of the epider- 

 mis of each is, at least in places, limited by a basement membrane which lies in 

 the structure itself and not at its base. These structures are, therefore, evagin- 

 alions of the entire body wall and cannot be considered as differentiations of 

 the epidermis alone. 



It follows from the above that everything contained within 

 the cuticula of the distal portion of the cephalic and anal cirri, 

 in the retractile tips of the palps, and in the tentacles and para- 

 podial cirri is in the epidermis ; therefore all the sensory cells in 

 these appendages, no matter how far they lie beneath the cuticula, 

 arc really situated in the epidermis itself, beneath the bodies of 

 the epidermal cells, it is true, but among their basal processes. 



In the cephalic cirri, the bodies of the cells of the diffuse 

 sense-organs lie anywhere between the bodies of the epidermal 

 cells and the axial nerve of the cirrus, but usually somewhat 

 nearer the former. In these appendages I have seen nothing 

 which can be interpreted as a sensory system of isolated nerve- 

 cells — all of the sensory -cells lie in definite gi^oups (Plate I, Fig. 

 I, Plate II, Fig. 33). The cells of any one group do not, how- 

 ever, always belong to a single sense-organ as is the case in the 

 epidermis of the body. Sometimes all the peripheral processes 

 from one group of sense-cells pass to a single modified area in 

 the cuticula and in such a case this group constitutes a single 

 sense-organ. Often the bundle of peripheral processes from a 

 single group of sense-cells, before reaching the bodies of the 

 epidermal cells, divides into two or three smaller bundles each 

 of which passes to a separate area in the cuticula (Plate I, Fig. 



