368 MOORE. [Vol. XIII. 



a mass of enlarged cells binding the tubules together, while 

 in Clepsine the protoplasm of the nephrostomal tubule has 

 completely included the accompanying recurrent and efferent 

 tubules. It will be recalled that in several species of disco- 

 drilids the efferent duct is completely buried in the plexus 

 mass. In Argilophilus the appearance of the ringed passages 

 would seem to indicate an actual growth around and inclusion 

 of the accompanying tubules by the nephrostomal tubule; but 

 here the cells have undergone division, though not so regularly 

 as in Hirudo. 



The apical tubule loops of Bdellodrilus appear at first sight 

 to resemble those of Lumbricus in arrangement, there being a 

 narrow tubule loop and a longer wide one with a slightly marked 

 apical ampulla. The succession of these two is, however, 

 reversed, the wide tubule leading to the efferent duct in Lum- 

 bricus and the narrow in Bdellodrilus. I cannot believe that 

 this will become explicable on any other than physiological 

 grounds. The efferent tubules correspond in a general way in 

 all three, but Clepsine lacks the terminal vesicle, while the long 

 muscular duct of Lumbricus is very much reduced in Bdello- 

 drilus, and the median dorsal pore of the latter is quite unique, 

 though the head kidneys of Lumbricus have a similarly placed 

 pore (34). 



The arrangement of the tubules of the discodrilid can be 

 derived from that of Lumbricus — if we omit the reversal in posi- 

 tion of wide and narrow tubules — in several ways, the most 

 obvious of which is by a great shortening of the recurrent limb 

 (apical lobe) of the latter, with the consequent withdrawal of 

 the connecting (recurrent) tubule from the plexus limb (main 

 lobe). This would result in carrying the basal end of the third 

 loop from the nephrostomal end to the distal end of the middle 

 loop, as in Bdellodrilus, while a close folding of the plexus limb 

 would result in the compact condition seen in all Discodrilidae. 

 The evolution has of course been in the opposite direction, the 

 blood supply to the nephridium, upon which the change partly 

 depends, being a later development. The mechanical effects 

 on the arrangement of the tubules, as a result of the manner 

 of attachment of the blood vessels, in restraining the free 



