Lang DON, Sense-organs of Nereis"^ virens. 23 



face views of an entire cirrus, the appearance of isolated cells. 

 In sections of the dorsal parapodial cirri, there is sometimes 

 found what appears to be a case of isolated sensory cells (Plate 

 I, Fig. 7). It should be noted, however, that each epidermal 

 cavity appears large for a single peripheral process ; that the 

 tip of the latter does not show any sign of branching into sev- 

 eral sensory hairs as would be necessary if a single cell supplied 

 one of the perforated membranes which, so far as I have been 

 able to determine, always contains several canals ; and that, in 

 one of the epidermal cavities figured, a second peripheral pro- 

 cess can be seen, although neither its peripheral end nor its cell- 

 body appears in the section. These sections were so faintly 

 stained that one could not determine whether or not other 

 processes were present in the epidermal cavities. In some sec- 

 tions of a dorsal cirrus stained with Kleninberg's haematoxylin, 

 the sensory cells appear plainly grouped into definite organs 

 (Plate I, Fig. 17). In my study of sensory hairs in living dor- 

 sal cirri, I have sometimes seen such an appearance as that fig- 

 ured in Plate I, Fig. 23. It will be seen that, near the base of 

 this cirrus, some of the sensory hairs appear to be isolated. But 

 in other dorsal cirri, all of the sensory hairs along the same 

 margin were clearly arranged in groups (Plate I, Fig. 18). It 

 must be either that all of the sensory cells of a dorsal cirrus — 

 except perhaps those supplying the extreme tip — are grouped 

 into definite sense-organs and the apparent cases of isolated 

 cells we owe merely to irregularities in the stain or to the de- 

 stroying of some of the sense-hairs; or else that cirri from the 

 same metameres in different worms or from different metameres 

 in the same worm vary — some having all of the cells grouped 

 into definite sense-organs and some having the cells at the base 

 isolated. From the evidence in hand I am inclined to the 

 former view. In the tip of both dorsal and ventral cirri, as seen 

 in living material, there always appears to be a few short isolated 

 sensory hairs. I have not yet obtained sections or mounts of 

 the removed cuticula which would enable me to verify this ob- 

 servation. In the tips of the cephalic cirri, which are homol- 

 ogous with the parapodial cirri, it can plainly be seen that the 



