84 T'r<iiix(trfi()iis. 



but passes outwards for a short distance between tlieni, and suddenly 

 turns back on itself to enter the j>osterior fold. The bridge canal is 

 ciliated, and this ciliated tube passes to the apex of the spur, where 

 it becomes suddenly much enlarged to form the '" ampulla '" (fig. 8, (Diq:)). 

 This is merely a .sudden and large increase in the diameter of the 

 bridge canal, marked, it is true, by the absence of cilia, and not otherwise 

 modified histologically as is the ampulla of Lvinhricns. 



From the ampulla at the apex of the spur the excretory canal turns bade 

 with slight decrease of diameter as the "wide tube"' (cj. This wide tube 

 forms a considerable ])a]t of the nephridium. and is very conspicuous in the 

 folds. It often contains globules of excretory fluid. Its course along the pos- 

 terior fold, through the dorsal lobe, and down the anterior fold is seen in fig. 8. 

 Meantime it has become gradually wider throughout its course from the apex 

 of the spur to the innoj' end of the anterior fold ; and, arrived at this latter 

 l>oint, it leaves the fold, turns back upon itself, and passes fi-eely outwards 

 towards the bladder as the outlet-duct (o.r/). The last region of the ex- 

 cretory tube, the bladder ((/), is a much dilated muscular sac with a wide 

 lumen and a thin Avail. It puts the nephridium in communication with the 

 exterior by the nephiidiopore, which is placed, as described above, at the 

 d(n-sal or ventral end of the sac according to the position of the nephridium 

 in the body. 



Fig. 8 is a drawing made up froni the study of a numbei- of nephridia. 

 for on account of their transparency the ducts could not be traced clearly 

 in any one nej^hridium unless the ])reparation were particularly fortunate. 

 In no two nephridia is the position of every region of the excretoiy tube 

 exactly the same in detail : the dorsal lobe especially shows great variation 

 in shape and in the details of coiling of the narrow tubes, while in the anterior 

 fold (me or other of these ducts is often completely hidden by the wide tube. 

 In the figure the ducts are represented as being faither away from each other 

 than is actually the case, for they could not otherwise be clearly shown. 



I may here revert to the structure of the first nephridium. A micro- 

 scopical examination proves it to be a nephridium without doubt, for there 

 is no development of the glandular tube at the expense of the other tube, 

 though all tube! are lengthened to a very great degree. The same regions 

 of the excretory duct {a, h, c, and d) are represented, apparently in the same 

 proportions as they occur in succeeding nephridia, except that the muscular 

 ])ladder is very much smaller in comparis(jn with the size of the nephridium. 

 it is impossible to unwind the folds, bound together as they are by inter- 

 lacing tubules, blood-vessels, and connective tissue ; but in all the folds 

 the two narrow canals are to be seen, winding in and out of each other in 

 parts in a manner recalling the dorsal lobe ; accompanying them is the wide 

 tube, with its usual straight course. In certain isolated j^arts of the nairow 

 tube cilia appear, and are seen in living specimens to be activel}" moving ; 

 while a single, rather wider, densely ciliated duct may be observed which 

 evidently represents the connecting bridge canal of the ordinary nephridium. 



The posterior end of the bladder in the first nephridium is three-lobed. 

 and one of the coils passes under this end, so that, with the outlet-duct 

 which enters the middle lobe, it looks as if the bladder received three 

 tubules ; but closer examination shows that two of these are the sides of a 

 U-shaped loop formed by this coil. 



Ciliaiion in the Canals. 

 It may be as well to emphasize the fact that, as in Laiiihricus, the cilia 

 ^.re restricted to a comparatively small part of the excretory tube. The 



