No. I.] SENSE-ORGANS OF LUMBRICUS AGRICOLA. 215 



{d) Around each nephridial opening, there is always a thick 

 cluster of small organs which may be called the nephridial 

 group. The sense-organs of this nephridial group are always 

 found around the nephridial opening, no matter zvhat its posi- 

 tio)i. The nephridial group is more prominent at the cephalic 

 end of the worm, and diminishes in size toward the caudal 

 end.^ 



It will thus be seen that the sense-organs are most numerous 

 and largest at the two extremities of the earth-worm ; that the 

 most prominent organs at these extremities are those of the 

 median zone; that those most prominent in the middle region 

 of the body are those of the cephalic zone; and that the degree 

 of prominence of the cephalic zone in any one region is inverse- 

 ly proportional to that of the median zone. If the nephridial 

 opening really had the definite position often assigned to it — 

 in front of the outer one of the inner pair of setae — a distinct 

 lateral line of larger organs might be traced along both sides 

 of the body. But the variability in the position of the nephri- 

 dial opening, not only from metamere to metamere, but also 

 on opposite sides of the same metamere, accompanied as it is 

 by a variability in the position of the nephridial group of sense- 

 organs and the position of the largest organs of the cephalic 

 zone, destroys all lateral symmetry in their distribution. The 

 cuticula from the ventral surface of the prostomium and from 

 the buccal cavity shows the cuticular markings of the sense- 

 organs irregularly distributed in both places, and occurring in 

 as great numbers on the ventral surface of the prostomium as 

 on its dorsal surface. The cuticular spots from the buccal cavity 

 are less distinct, owing to the fact that the cuticula in this 

 region is not elevated over the summits of the sense-organs. A 

 groove often surrounds the cuticular spot, the same groove 

 noted in sections of the sense-organs from the buccal cavity. 



In a worm about 19 cm. long, which contained 153 meta- 

 meres, there was found to be an average of 1000 sense-organs 



1 It is difficult to explain the presence of this group around the external open- 

 ing of a ncphridium unless it serves as a guard against parasites. The cuticular 

 spots are exactly like those found over the rest of a metamere, and sections show 

 that the underlying sense-organs have the same structure as those found else- 

 where. 



