TUBIFEX. 341 



THE EXCRETORY ORGANS. 



Tiie excretory organs or uepliridia consist of a 

 system of paired tubes, which are present in most 

 segments of the body. They are absent from a few of the 

 anterior segments, but as in most aquatic Oligochaeta 

 they begin well before the genital segments, from which, 

 however, they are absent. I have been able to trace these 

 uepliridia as far forward as segment 7, but not in front 

 of that. Behind the genital segments they are to be 

 found through all the remaining segments of the body. 



The nephridia are coiled tubes occupying two 

 segments, and, typically, two only, but in a few cases 

 I have seen the coils of a nephridium of one segment 

 lying in the segment behind, but this must be considered 

 as an abnormal condition. Each nephridium is provided 

 with an internal and an external aperture, the former 

 opening into the body cavity, the latter to the exterior. 

 The nephridia lie nearer the ventral surface of the body 

 than the dorsal, and are situated one on either side of the 

 ventral vessel and very close to it. In the posterior 

 segments of the body certain of the coils of the tube 

 appear to be very intimately connected with this vessel, 

 so that even though the movements of the worm may be 

 extremely violent, these tubes do not become displaced. 



Nasse (1882), in his work on the family Tubificidae, 

 described the presence of branches of the ventral vessel 

 which arise near the nephridia and pass into close 

 connection with these organs. That is, he claims that 

 there are special blood vessels conveying blood from the 

 ventral vessel to the nephridia, in which organs, pre- 

 sumably, the blood is purified. Nasse does not explain 

 how the blood is returned to the main circulation, but he 

 suggests, at any rate, an arrangement of the vessels and 



