No. 3.] THE DISCODRILID NEPHRIDIUM. 365 



but little ; the chief points of variability, other than certain 

 special developments, are in the presence or absence or relative 

 development of the muscular efferent duct, the degree of com- 

 plexity of the funnel, the relative prominence of the middle 

 and third loops, — both of which are always present, and the 

 latter always to be identified as a group of four parallel and 

 usually conspicuous tubules, — the extent to which the con- 

 necting tubule is associated with ■ the proximal limb of the 

 middle loop, the arrangement of the cilia, the presence or 

 absence of an ampulla on the distal bend of the wide tubule 

 loop, and the extent and character of the branching of the 

 lumen. 



The latter is of special interest, as it has been only a few 

 years since such plexuses were regarded as characteristic of 

 the leech nephridium. Since Bourne's discovery in Hirudo (10), 

 branchings of the lumen have been described for a great many 

 Oligochaeta, and this too in many of the simplest nephridia. 

 The plexus is always developed at about the same region, but 

 it is interesting to note the great variability in extent and in 

 the manner of branching of the lumen. In some Naidae (Vej- 

 dovsky, 33) and Tubificidae (Stole, 32) the branching occurs 

 within the confines of a single cell, in others within a number of 

 such cells massed together by the folding of the tubule ; from 

 this all degrees of development of the plexus are found, up to the 

 remarkable conditions presented in such forms as Pontodrilus 

 (Eisen, le), in which the tubules themselves as well as the 

 lumina branch, and the very curious and instructive arrange- 

 ment presented by Argilophilus (Eisen, 17), in which the con- 

 tinuous but very irregular lumen gives off in part of its course 

 numerous ring-like diverticula, which completely encircle the 

 accompanying recurrent and efferent tubules. The plexus 

 which Bolsius (s) has described in the enchytraeid nephridium 

 is perhaps of a different character, and has been formed by 

 secondary anastomoses of contiguous parts of the winding 

 lumen, rather than by longitudinal division of the drain-pipe 

 cells with the resulting reticulation of their lumina, as in 

 Hirudo, Pontodrilus, etc. Now the chief point to be noted is 

 that whatever the degree of variation in other respects, the two 



