NEPHRIDIA OP CERTAIN EARTHWORMS. 399 



posterior segments they tend to be arranged in accordance 

 with the number of the setae, i. e. eight per somite. This dis- 

 position of the nephridiopores is associated with a correspond- 

 ing regularity of the nephridial tufts, which tend to break up 

 in the posterior segments into eight separate nephridia. There 

 is, however, a connection between them, as shown in figs. 2, 3. 

 A series of tubules (figs. 1 n and 5 n) run from seta to seta, and 

 unite the nephridial tufts of these setae ; this tubule for the most 

 part runs within the peritoneum. 



The appearances presented, in fact, can hardly be explained, 

 except on the assumption of a network of nephridial 

 tubules. In no Annelid, except in Pontobdella (Bourne 

 5) has the nephridial system been recorded to be a network. 

 I can, however, find no evidence that the nephridial network 

 of one side of the body is continuous with that of the other ; 

 nor does there appear to be any connection between the 

 nephridia of successive segments. 



The tubules leading to the exterior are, in the posterior 

 segments, nearly invariably related to the setae ; occasionally 

 (fig. 2fl) a tubule was observed to perforate the body wall 

 between the setae, but in this case it must be noted that the 

 tubule passes up a septum of connective tissue, which at this 

 point breaks the continuity of the longitudinal muscle laver. 

 It may be that such septa, which occur here and there, represent 

 the last trace of setae which have now disappeared. Sometimes 

 (fig. 3 a a') I noticed two nephridial tubules belonging to the 

 same seta. I did not succeed in observing with certainty whether 

 or not they opened on to the exterior by distinct orifices. The 

 ductule is usually embedded in the lax connective tissue which 

 surrounds the insertion of the seta (fig. 4) ; it passes up as far 

 as the circular muscular layer without undergoing any changes 

 in character, that is to say, the duct is intracellular. At 

 this point the tubule often runs for some distance along 

 the junction between the circular and longitudinal muscles 

 before perforating the former, aud opening on to the exterior ; 

 in other cases (fig. 4 b) the tubule at once perforates the circular 

 muscular layer and the epidermis, and opens on to the exterior. 



VOL. XXVIII, PART 3. NEW SER. E E 



