88 THE VOYAGE OF H.M.S. CHALLENGER. 



of Schizonemertea (PL XIV. fig. 2) I could observe that from the thickened part of the 

 plexus, which forms the longitudinal stem, fine nerve-branches also take their origin, and 

 pass directly to the periphery. The essential difference between Schizonemertea and 

 Hoplonemertea in this respect nevertheless remains the same as above formulated. 



We may now turn to the Hoplonemertea. The different species contained in the 

 Challenger collection confirm the well-known facts about the metamerically placed pairs 

 of peripheral nerves of the Hoplonemertea, some of which are turned dorsally, others 

 ventrally, and which, dividing dichotomously, finally spread out in very numerous 

 bundles of nerve-fibrils, serving for the innervation of the environing regions. In addi- 

 tion they also furnished me with certain important new points. To begin with the latter, 

 I will first draw attention to the two longitudinal nerve-stems of Drepanophorus 

 lankesteri, which are situated, as is characteristic of this genus, below the intestinal 

 caeca (PI. IX. figs. 1, 5, 6), about midway between the lateral margin of the body and 

 the median ventral line. These stems in transverse section very much resemble those 

 of other Hoplonemertea. One of them is figured on PI. XII. fig. 5 ; from this it may be 

 seen that the nerve-cellular coating is generally not distributed as a sheath all round the 

 fibrous core, but as a double band applied upon this core at two diametrically opposite 

 points. The participation of this cellular coating in providing the outgoing peripheral 

 nerves with delicate nerve-fibres is distinctly seen in this section, as is also the direct 

 continuity of other portions of the peripherally directed nerve-fibres with those of the 

 core. 



When in Drepanophorus lankesteri I followed some of these peripheral nerves in 

 their further course, by examining the consecutive sections in which they are continued, 

 I was struck by the very remarkable fact, never noticed before, that some of them 

 did not dichotomise — or at least very rarely — and did not taper towards the periphery, 

 but passed directly under the intestine from the one longitudinal nerve-stem into the 

 other, a distance in this specimen of 1^ mm. This was an unmistakable com- 

 missure, which could in no way be compared to the well-known commissure above 

 alluded to, which connects the two longitudinal stems above the anus. And not 

 only was one such ventral commissure present, but on closer inspection I found a 

 great number of them, and by registering the respective distances at which they 

 were present, the one behind the other (about ^ mm.), I was forced to the conclusion 

 that we here have before us a system of very regular metamerically placed commis- 

 sures between the longitudinal stems, and forming a nerve-ladder (PL IX. fig. 10), 

 which is very directly comparable to that of Sabella and other species among Annelids, 

 and to that of Proneomenia and Chiton anions; Molluscs. In a few of these commissures 

 I detected dichotomy and fusion of one of the branches thus formed with the fore- 

 going or with the following commissure, a peculiarity also known to exist in Chiton, but 

 evidently of rarer occurrence in Drepanophorus lankesteri. Moreover, I may also men- 



