2 14 LANG DON. [Vol. XI. 



1. The sense-organs are largest and most numerous in the 

 prostomium and first metamere. Here they are irregularly dis- 

 tributed, so that there is no evidence of a distinct grouping. 



2. Distinct groups or zones of sense-organs may be recog- 

 nized over the rest of the body. It should be borne in mind 

 that every metamere contains sense-organs irregularly scattered 

 over it outside of these groups and zones. The latter are 

 merely noticeable from the fact that the organs in them are 

 larger or more numerous than elsewhere. In the metameres 

 back of the first the following arrangements of the sense- 

 organs may be noted (PI. XIV) : 



{a) There are always more sense-organs in the cephalic than 

 in the caudal half of a metamere. This is also true of the first 

 metamere. 



(b) In the fifth or sixth metamere, the organs are seen to 

 be more numerous in a zone which passes around the cephalic 

 margin of the metamere. Passing caudad, this ccpJialic zone 

 increases in prominence for six or seven metameres. It then 

 becomes reduced to a single irregular row of large sense- 

 organs, and remains as such until near the caudal end of the 

 worm. In the caudal metameres, it gradually loses its promi- 

 nence by a decrease in size of its organs. In this cephalic 

 zone the smallest organs are generally found on the dorsal sur- 

 face, and the largest near the nephridial opening. 



(f) In the second metamere, there appears a zone of some- 

 what larger organs, which occupies the elevated caudal border 

 of a slight groove that always passes around each metamere 

 just cephalad of its median circumference. This zone is in 

 line with the setae, and may be called the median zone. In 

 the third and fourth metameres this median zone is well, 

 marked. From this point on, it diminishes in width until, 

 back of the clitcllum, it is reduced to a single irregular row of 

 large organs. Continuing caudad, the organs of the median 

 zone are found to diminish in size until about the last seventh 

 of the body is reached. They then increase in size to the 

 caudal end of the body. As a rule the smallest organs in this 

 median zone occupy its dorsal part, and the largest ones are 

 near the setae. 



