330 MOORE. [Vol. XIII. 



and Branchiobdella instabilia) which were examined with refer- 

 ence to this point. It is of interest that certain localities seem 

 to furnish individuals predominantly of one kind, a fact which 

 will be further developed in another connection. 



In those species which have well-developed inter-segmental 

 septa, as is the case in Bdellodrilus philadelphicus, Branchiob- 

 della instabilia, pulcherrima, and parasita, the opaque masses 

 are strictly confined to their proper somites. On the other 

 hand, those of B. illuminatus, in which the septa of the seg- 

 ments anterior to the genital region are imperfect or altogether 

 aborted, are more variable in position. The anterior one may 

 lie partly or wholly within the third somite, and the posterior 

 partly or wholly within the fourth, or both may lie — but this 

 is a rare condition — symmetrically within the third. ^ In this 

 species the greater number of individuals examined up to the 

 present time had the anterior nephridium to the left, the 

 posterior to the right of the intestine. 



The general arrangement of the tubules can best be seen 

 when living worms are examined under a low power. For 

 convenience, let us suppose an example of B. instabilia in which 

 the right nephridium is anterior — but the windings have prac- 

 tically the same form in all species. Three somewhat complexly 

 looped tubules pass around the margin of the compact granular 

 mass from the ventral to the dorsal anterior border, where they 

 turn backward and are joined by a fourth, which appears from 

 within the opaque mass. The four are now arranged in two 

 pairs, of which one, composed of two of the tubules which arise 

 from the ventral margin, forms a longer loop, and is twisted 

 and folded about the other comparatively straight one. The 

 four tubules, lying side by side, now leave the opaque mass at 

 its posterior dorsal region, bend to the left, and extend trans- 

 versely over the intestine, in more or less close contact with 

 the septum "/„„ to the left body-wall, to which they are 

 anchored by their peritoneal investment. Turning sharply 

 forward they extend in a loose, wavy loop to the anterior left 

 angle of the same somite, and are here once more anchored to 



1 One example of the anterior opaque masses, both lying in the third somite, 

 was observed in B. instabilia. 



