Vol. l] Bovard. — Sense Organs in Microscolex Elegans. 275 



The total number of sense organs in a single Lwmbricus was 

 approximated by Miss Langdon ('95) at 150.000, while for a 

 Microscolex of 103 segments only 14,787 were found. Thus 

 Microscolex, with approximately one-fourth the surface of Lum- 

 bricns, has only one-tenth as many sense organs. 



The largest sense organs in Microscolex (35 microns in 

 diameter) arc on the prostomium, and the smallest (6 microns) 

 on the clitellum. For the rest of the worm, the organs have 

 about the same average size (15 to 19 microns). Those of the 

 posterior end do not increase in size as do the numbers. In 

 Lumbricus, on the other hand, the largest are on the prostomium 

 and the first metamere, and eaudad they gradually diminish in 

 size, the smallest being on the caudal segments. 



Besides these differences in distribution over the general 

 surface of the worm, there are certain zones in each metamere 

 where there is a perceptible grouping of the sense organs. This 

 distribution differs both antero-posteriorly and dorse-ventrally. 

 In the antero-posterior direction three zones are to be seen. The 

 first, the cephalic or anterior zone, extends from the septal fur- 

 row in front of the median zone of the metamere. The surface 

 of the anterior zone is an are. the convexity of whose surface 

 points in the anterior direction. The nephropores occur in this 

 zone close to the septal furrow. The largest numbers of sense 

 organs are in this zone, and passing eaudad they decrease 

 in size and numbers in the successive meta meres. For the most 

 part the organs of this zone are most numerous near the septal 

 f m-row. They do not occur in a distinct line along the anterior 

 margin, as in Lumbricus, but are scattered over a small area or 

 belt. The numbers become fewer as we approach the succeeding 

 zone, making a gradual transition from a region of large to one 

 of comparatively small numbers. 



The second zone is a narrow one extending through the 

 middle of the metamere. and consisting of a single row of the 

 largest sense organs found in line with the ehaetae sleeves. This 

 is true in all the metameres except the first, where the largest 

 organs are in the anterior zone. It is impossible to make out 

 any distinct groupings about the ehaetae sleeves, as was shown 

 in the ease of Lumbricus by Miss Langdon ('95). 



