34 Journal of Comparative Neurology. 



caudal metameres are uniformly level and their sense-organs 

 have, therefore, a uniform distribution ; in metameres 6-20 the 

 ventral line is depressed and the sense-organs are, therefore, 

 fewer in the mid-ventral line and more numerous on the raised 

 areas on either side. 



The preceeding observations enable one to formulate the 

 following general law of distribution: The juimber of sense-organs 

 with wJiicJi a given portion of the body is supplied is directly 

 proportioJial to the degree to which this surface is exposed to me- 

 chanical stimuli — that is, to the extent of its elevation above sur- 

 rounding parts. 



The distribution of the diffuse sense-organs in the various 

 parts of the body in accordance with the preceeding law may 

 be summarized as follows : 



1. Numerous sense-organs are irregularly scattered over 

 the entire surface of all the special tactile appendages — the 

 cephalic, anal, and parapodial cirri, the retractile tips of the 

 palps, and the tentacles. (See also Plate I, Figs. 19 and 21 

 and Plate III, Fig. 51.) 



2. Over the bases of the parapodia and of the cephalic 

 and anal cirri and over the gill lobes the organs are few in num- 

 ber. The dorsal surfaces of the enlarged bases of the palps are 

 supplied with a very large number of organs ; the ventral sur- 

 faces contain a very small number. 



3. The dorsal surface of the prostomium contains num- 

 erous irregularly arranged organs — about 400 in all — of which 

 those on the anterior portion are fewer and smaller. On the 

 ventral surface of the prostomium the organs are smaller and 

 fewer in number. 



4. The peristome contains a very large number — about 

 3600 — of sense-organs. These organs are irregularly scattered 

 over the surface in broad irregular bands which pass obliquely 

 cephalo-caudad across the peristome and are limited by the 

 grooves which mark its surface. The sense-organs are more 

 numerous on the dorsal and larger on the ventral surface. They 

 are both larger and more numerous around the anterior than 

 around the posterior part. Around the anterior margin of the 



