520 OLIGOCHAETA 



but in Acanthodrilus the nephridia are as invariably paired. It is, indeed, upon 

 these characters that the genera of the Acanthodrilidae are distinguished. In the 

 genus Acanthodrilus the paired nephridia in a few species (e.g. A. novae-zelandiae 

 and A. dis&imilis) alternate in position from segment to segment ; this is also 

 found in Plagiochaeta; the alternation of the nephridia is coupled with a difference 

 in structure ; I have, however, already gone into this matter at some length (p. 38), 

 and need not, therefore, return to it again. It may, nevertheless, be pointed out 

 that it is only among the New Zealand species that this peculiarity is met with. 

 Where the nephridia are diffuse, the external apertures are, of course, not visible ; 

 they are too minute to be detected by the unarmed eye. There is generally an 

 accumulation of nephridia in the anterior segments ; in Octochaetus, and doubtless in 

 other species, when they come to be better known, the same thing will be found ; 

 the anterior nephridia form a compact mass which opens by a duct into the 

 buccal cavity. This state of affairs is not peculiar to this genus, but occurs 

 in other Oligochaeta. 0. multiporus shows one peculiarity, at present unique in 

 the Oligochaeta ; in a few of the posterior segments the nephridia are particularly 

 abundant, and open, not only on to the exterior by numerous pores in each segment, 

 but also into short diverticula of the hind gut. In this region of the body the 

 nephridia are, exceptionally, provided with funnels. 



The vascular system of the Acanthodrilidae is constructed upon the same plan as 

 is that of most other terrestrial Oligochaeta; unfortunately there is no published 

 account of the vascular system derived from a study of the living worm, such as 

 we possess of Megascolex and Pontoscolex ; the dorsal vessel is occasionally double, 

 a condition which occurs in other Oligochaeta ; the doubling of the dorsal vessel is 

 chiefly met with in the New Zealand Acanthodrilidae, but not in all of them ; it 

 characterizes 0. multiporus, 0. huttoni, 0. thomasi, and 0. antarcticus; in these 

 species the dorsal trunk is completely double from end to end of the body ; in 

 A. novae-zelandiae and in A. rosae the tube is double, except where it passes 

 through the septa ; at these points the two tubes become united. Above the 

 oesophagus runs a supraintestinal trunk, which is connected with the posterior of 

 the peri-oesophageal vessels ; of these there are a number of pairs, and usually the 

 last four of these are larger than the others, sometimes there are only three of these 

 intestinal hearts ; when there are four the last lies in the thirteenth segment ; they 

 are connected with the dorsal as well as with the supra-intestinal vessel ; there is 

 often, perhaps always, a pair of lateral vessels running along the body-wall in the 

 anterior segments ; according to HOBST these vessels arise from the dorsal vessel, 

 instead of arising as do their homologues in Perichaeta from the oesophageal plexus. 



