THE ANATOMY. NEPHRIDIA 37 



often figured as composed of a mosaic of numerous cells. It is, however, occupied by 

 only a single large crescental cell. 



The 'narrow tube' which follows the funnel is 'the largest part of the whole 

 nephridium ; the lumen is wide and of course intracellular l ; here and there the lumen 

 shows slight indications of branching. Cilia are not universally present, but there 

 is an alternation of ciliated with non-ciliated tract. This is followed by the ' middle 

 tube,' which is of less extent and ciliated throughout ; its calibre is greater and the 

 walls are more glandular. 



The ' wide tube ' commences with a wider dilatation where it communicates with 

 the middle tube. This part is also very glandular, but not ciliated. The last section 

 of the nephridium is formed by the muscular duct. This has apparently a lining 

 of large cells, so that the duct is intercellular ; it has numerous muscular fibres 

 in the walls. This part of the tube opens directly on to the exterior. The actual 

 course followed by the windings of the nephridium will be apparent from the 

 illustrations and a special description is unnecessary. 



Nephridia of other Genera. Lumbricus is really the only genus of earthworms 

 whose nephridia have been carefully studied from the point of view of their minute 

 structure ; there are, however, a few details to hand, which have been for the most 

 part Collected together in BENHAM'S paper. In all the earthworms with paired 

 nephridia the same regions of the tube can be distinguished ; but frequently there 

 are differences in the relative development of the various parts. This is particularly 

 the case with the terminal muscular section. In many genera of earthworms this 

 section is very wide and large in proportion to the rest of the tube ; especially 

 is this the case for example with Acanthodrilus dissimilis and a few other species 

 of that genus, and with Microchaeta, etc. Moreover, in the species named, the 

 muscular duct of the nephridium has a caecum given off near to its external pore ; 

 the presence of such a caecum is very common. 



On the other hand, there are a good many species which appear to be without 

 the terminal muscular duct, or in which at any rate it is but slightly developed ; 

 this seems to be especially the case with small forms ; and it is perhaps a mark 

 of degeneration ; such genera as Gordiodrilus and its allies show an apparently 

 complete absence of the muscular sac. The above remarksj it will be understood, 

 refer only to the genera with paired nephridia ; in those with diffuse nephridia the 

 terminal sac seems to be invariably wanting. The funnel too shows a certain amount 

 of variation; it is larger or smaller as the case may be; in Rldiwdrilu* 



1 VEJDOVSKY (9, p. 349, etc.) does not admit intracellular nature of duct ; he believes it to bo intercellular 

 throughout. 



