Langdon, Sense-organs of Nereis virens. 35 



ventral surface is a comparatively wide band entirely destitute 

 of sense-organs. 



5. That part of the buccal cavity which corresponds in 

 position to the peristome and which is protruded when the ani- 

 mal is feeding contains a small number of very large sense-or- 

 gans. Their distribution is practically the same as that in the 

 peristome. 



6. In the first metamere the sense-organs are more uni- 

 formly distributed than in the peristome. They are somewhat 

 more numerous on the dorsal than on the ventral surface and 

 are less numerous around the parapodia. 



7. Passing caudad from the first metameres the following 

 changes in distribution take place : 



a. In each succeeding metamere, the organs become very 

 gradually fewer in number up to and including the last caudal 

 metamere. In metamere 41 there are only 11 00 organs as 

 against 2000 in the first metamere. In a few of the anterior 

 metameres this decreases in number is accompanied by a de- 

 crease in size. 



b. There are more sense-organs on the ventral than on the 

 dorsal surface in a few anterior metameres. This difference 

 becomes gradually less to about metamere 12 at which it dis- 

 appears. 



c. The sense-organs extend close to the posterior border 

 of each metamere but are lacking over a narrow zone extend- 

 ing around the extreme cephalic border. 



F, Function. 



I do not find in the literature any experimental work on 

 these sense-organs, neither have I been able to carry on such 

 work myself. The parapodial, anal, and cephalic cirri, the ten- 

 tacles and the retractile tips of the palps, from the way in which 

 they are used, have always been regarded as tactile. Nereis 

 possesses true eyes and other paired cephalic organs which 

 probably serve for other special senses ; it therefore seems to 

 me that all of the diffuse sense-organs probably belong to the 

 class of tactile organs. Those situated in the appendages and 



