Langdon, Sense-organs of Nereis virens. 5 3 



are apparently confined to the first 20 metameres. The only 

 appendages in which they are found are the gill lobes and the 

 bases of the palps and cephalic cirri. 



A reconstruction of the head and first metamere shows 

 that in this region the spiral organs have a definite arrangement 

 which may be summarized as follows. (Plate I, Fig. 28): 



1. On the base of each palp about 43 organs are scat- 

 tered over the outer or lateral surface ; they are entirely lacking 

 on the inner surface. 



2. A few organs are found in the outer surface of the 

 bases of the cephalic cirri. 



3. On the dorsal surface of the prostomium are two broad 

 irregular bands of 15 or 16 organs each, which extend, one on 

 either side, from the base of the tentacles almost to the anterior 

 eyes. Each of these bands is thus practically in line with the 

 true eyes on its own side of the prostomium. No spiral organs 

 are found on the ventral surface of the prostomium. 



4. About 372 organs occur in the peristome in the raised 

 surfaces between the grooves. In the mid-ventral region these 

 organs extend to the posterior margin, but not to the anterior 

 one. Passing dorsad they may be said to be distributed in a 

 wide band which occupies about the median two-fourths of the 

 peristome. In the mid-dorsal region, this band is narrow and 

 is found only at the anterior border. Just caudad of the base 

 of the cephalic cirri this band is extended to the anterior mar- 

 gin by a group of about 33 organs. 



5. In the first metamere about 56 organs are found in a 

 narrow median zone which passes over the dorsal and lateral 

 surfaces between the parapodia of opposite sides. Just ventrad 

 of each parapodium is a group of 8 or 10 organs. About 6 

 organs are found in the anterior half of the mid-ventral region 

 of this metamere. 



Although the above statements rest on a study of but one 

 reconstruction, my study of sections of other worms shows 

 that the main facts of distribution must be generally true. Var- 

 iations in the whole number and exact location of each organ 



