38 OLIGOCHAETA 



the funnels of the anterior nephridia are very large, and this condition is rather 

 characteristic of the family Geoscolicidae ; it occurs also in Pontoscolex, where I have 

 described it myself. In the last-mentioned genus the funnel is followed by a very 

 wide section of the ' narrow tube.' The funnel varies in size in different worms, 

 but in no earthworm with paired nephridia is it totally absent. 



The occasional branching, or rather the indications of branching, observable 

 in the nepbridium of Luinbricus have already been referred to ; Bbnham has 

 described in Microchaeta a complicated branching and anastomosis of the fine tube 

 carried to such an extent that it formed a network round the other regions of the 

 tube ; more recently KosA has met with the same thing in the genus Desmogaster ; 

 in Eudrilus I have seen a certain amount of branching, but not so developed as in 

 the genera mentioned. In all cases the nephridia of the higher Oligochaeta have 

 an abundant blood supply ; this runs of course in the peritoneum which invests the 

 nephridia externally; the only genera in which this vascular supply is absent (or, 

 at the most, feebly developed) are Ocnerodrilus and Gordiodrilus. The actual course 

 of the vessels supplying the nephridium will be described under the vascular system. 



Specialization of nephridia. Another matter which is worthy of note in 

 connexion with the paired nephridia is the specialization which is occasionally 

 shown in different regions of the body. Among the Geoscolicidae it is common for 

 a variable number of pairs of nephridia, occupying the anterior segments of the 

 body to differ in structure from those which follow and occupy the rest of the 

 body. The very first pair of all, in Rhinodrilus ecuadoriensis for example, but 

 ,also in worms belonging to other families, often appear to have acquired a different 

 function for they open into the buccal cavity ; but the consideration of these is 

 deferred to a subsequent page. I am now concerned with those cases which 

 may be exemplified by Microchaeta. In that worm the nephridia down to about 

 the twenty-seventh segment are furnished with a long oval caecal appendage to the 

 terminal sac. In the nephridia from the twenty-eighth segment onwards the terminal 

 sac is larger and wider and is prolonged beyond the external orifice ; this prolongation 

 corresponds of course to the caecum in the anterior nephridia, but it is hardly 

 marked and is a continuation of the sac not being bent back upon it. 



In Acanthodrilus novae-zelandiae, one or two allied species, in Gryptodrilus fletcheri, 

 and a few other earthworms, there is a very remarkable specialization of the nephridia, 

 not connected as in Microchaeta with the cephalization of the anterior segment. 

 There are in the worms now under • consideration two series of nephridia which 

 open on to the exterior either in relation to the ventral or to the dorsal setae ; but 

 although there are two series there is only a pair of nephridia in each segment. 



