Langdon, Sense-organs of Nereis virens. 33 



of the wall. I can also find cells which show all gradations 

 from one of these conditions to the other. 



I can, therefore, at present only offer the two following 

 statements : 



1. If the appearances described above are pericellular 

 nerve-baskets, some, if not all, of them are the final termina- 

 tions of the central processes from the diffuse sense-organs. 

 The fibers of these baskets must then strengthen the wall of 

 the ganglion cells with which they are in contact so that these 

 parts, upon the wrinkling of the cell wall become the summits 

 of folds, and the apparent baskets found in other cases — i.e., 

 on varicosities — must be artefacts which further study will en- 

 able one to clearly distinguish from the true structure, 



2. If the appearance described above is not a pericellular 

 nerve-basket, it is due to the formation of an artificial network 

 in or on the wall of the ganglion cell and this network must 

 differ chemically from the wall so that it takes the blue stains 

 with greater readiness then does the wall itself. 



E. Distribution. 



A study of the distribution of the diffuse sense-organs by 

 means of the modified cuticular areas as they are seen in a sur- 

 face view of the removed cuticula and a comparison of the re- 

 sults of this study with the form of the body of Nereis reveals 

 several suggestive facts. (See Plate III, Fig. 50.) The ante- 

 rior end of the body, which is naturally more exposed to con- 

 tact, is supplied with a very large number of sense-organs. All 

 portions of the surface that lie in grooves and are thus protected 

 by their position are entirely destitute of sense-organs. Those 

 portions that are protected by a position near special tactile ap- 

 pendages are sparingly supplied with sense-organs ; for instance 

 the organs are very numerous over the cephalic metameres but 

 become very few on the caudal ones because the caudal end is 

 so narrow that both dorsal and ventral surfaces are protected by 

 the relatively large parapodial cirri. Those portions of the 

 body that have a uniform surface have a uniform distribution of 

 their sense-organs ; both the dorsal and ventral surfaces of the 



