116 THE VOYAGE OF H.M.S, CHALLENGER. 



XIII. figs. 7-9), although, when these walls are distended by the contents, theii- 

 thickness correspondingly diminishes (PI. XIII. fig. 8). In this species, moreover, it 

 could be observed that towards the anterior end of this massive nephridial canal it 

 subdi\'ides into smaller branches (PL XIII. fig. 9), applied against the wall of the blood 

 lacuna, and nearly escaping observation amongst the epithelial cells of these lacunae. 

 Here, again, no internal opening could be demonstrated ; whereas, at the posterior end, 

 the nephridial canal, making a sharp curve, passes outwards above the nerve-stems. 

 We thus observe difierences in the nephridial system of the Schizonemertea, which may 

 be classified under the following heads : — 



(a) The nephridial canal may be massive and single throughout the greater part of 

 its course, only ramifying anteriorly [Cerehratulus macroren, &c). 



{h) It may be very copiously subdivided, every transverse section showing a consider- 

 able number of lumina, the whole thus forming a kind of network, with certain principal 

 longitudinal ducts, and being suspended against the wall of the circumoesophageal blood 

 lacuna (EiipoUa). 



(c) The meshes of this network may be situated in the region between the probo- 

 scidian sheath and the longitudinal nerve-stems (most Cerehratidi) , or they may stretch 

 ventrally to these nerve-stems {Eu2MUa). 



(d) The ducts leading to the exterior may be one on each side, and these generally 

 terminal (posteriorly). 



(e) There may be two on each side, and they may then communicate with the chief 

 longitudinal canal about its middle. 



{/) They may be more numerous, often in various phases of distension, and arranged 

 more or less metamerically. The histological characters are fairly uniform, the lumen is 

 never excavated in a row of cells as is the case in so many Platyelminthes, in the 

 Discophora, &c., but is always bounded by a certain number of cells in every section. 

 Each of these cells has a large and distinct nucleus ; its protoplasm is granular, and the 

 free surface, which is turned towards the lumen, is ciliated. 



We have now to examine the nephridia of the Challenger Hoplonemertea. In 

 Oudemans' paper several points concerning them have been already noticed ; other details 

 referring to the situation and the number of the deferent ducts were mentioned above 

 when the species were described. 



Hence it may here be suflacient to call attention to the fact, made specially palpable 

 by certain of the Challenger preparations, that whereas the nephridia of the Hoplone- 

 mertea are no longer suspended in blood lacuntQ, but wholly surrounded by the gelatinous 

 tissue, they are at the same time much more intricately coiled and ramified, as 

 can be very easily seen from a comparison of fig. 1 {Nep), PI. X., with figs. 7, 9, PI. 

 XIII. This involves, however, no important change in the histology; what we have 

 noticed under this head in the Schizonemertea holds good for the Hoplonemertea as well. 



