NEPHRID1A OP CERTAIN EARTHWORMS. 407 



duct, one on each side of the body. Finally, the disappearance 

 of the longitudinal duct leaves the paired nephridia of the 

 majority of Annelids. The remarkable disposition of the 

 nephridia of Pontobdella, described by Bourne (5), is dif- 

 ferent in that there is not a single longitudinal duct on each 

 side, but a continuous network. It resembles, in fact, as Lang 

 has pointed out, the branched and anastomosing canals of the 

 Trematode nephridium ; it must also be remembered that the 

 longitudinal canals of Gunda segmentata break up here 

 and there into a network of anastomosing canals. It is this 

 condition which has been inherited by Pontobdella, and 

 also by Pe rich aeta ; Perichaeta appears to me to represent 

 a more archaic structure than Pontobdella since the external 

 pores are more numerous; in Pontobdella they have become 

 reduced to a single pair in each somite. Acanthodrilus 

 multiporus offers the next stage; here the nephridial net- 

 work is not continuous from segment to segment, but there is 

 a separate network for each segment opening by numerous 

 external pores. The Capitellidae (Eisig 7) agree in many particu- 

 lars with Acanthodrilus; the nephridia of each segment are 

 numerous, and occasionally are connected ; the ducts opening 

 on to the exterior are often (Fischer 8) branched and open by 

 several distinct pores. In the posterior region of the body 

 of Acanthodrilus the external apertures are fewer, and are 

 more particularly connected with the setae, while the nephridial 

 network tends to become arranged in a series of tufts, corre- 

 sponding in number to the setae ; finally, the nephridial net- 

 work of each half of the segment is independent. From 

 Acanthodrilus to Lumbricus there is a considerable gap, 

 only very partially bridged over by such forms as Plutellus 

 (Perrier 12), where the irregularity in the position of the 

 nephridial pores is, perhaps, to be regarded as a last trace of 

 the numerous nephridial pores of Acanthodrilus and 

 Perichaeta. The above considerations necessitate a further 

 inquiry into the nature of the longitudinal canal in Lauice, 

 &c. ; if it be assumed that this latter is the homologue of the 

 longitudinal canal in the Platyhelminths, it must follow that the 



