REPORT ON THE NEMERTEA. 89 



tion that in certain commissures it was clear that fine nerve-twigs spring from them and 

 serve to innervate the surrounding tissues, their significance thus not being solely com- 

 missural. The different facts just recorded are represented semi-diagrammatically on PI. 

 IX. fig. 10, which was reconstructed from the very large number of sections which I 

 have of this species. It is, moreover, seen in this reconstruction how other peripheral 

 nerves spring from the longitudinal stems as well, some being directed upwards or 

 downwards, some towards the side or inwards. These peripheral stems are metamerical, 

 as are the commissures, a metamery which, though not absolute, and sometimes broken 

 by certain irregularities, is still more advanced towards perfect regularity than is the 

 incipient metamery which we observe in the nerve-tracts that are noticed inside the 

 plexus of the Schizonemertea, and that were more fully described above. The transverse 

 commissures between the lateral stems may be noticed to go up quite close to the brain- 

 lobes, as indicated in the diagram on PI. IX. fig. 10. Both in this respect and in the 

 fact of their existence, they call to mind the ventral commissural tracts in the plexus of 

 Schizonemertea. I have no doubt that the two systems are homologous, the commissures 

 having subsisted in Drepanophorus lankesteri although the plexus has disappeared. 

 Finally, it must be mentioned that as yet I have looked for them in vain in other species 

 of Drepanophorus, or in other Hoplonemertea. My other specimens of Drepanophorus 

 are, however, less well preserved than is the one specimen of Drepanophorus lankesteri. 



Another peculiar feature of the peripheral nerve-system of the Challenger Hoplo- 

 nemertea, which has also remained hitherto unnoticed, is most favourably observed 

 in Amp>hiporus marioni, although I afterwards noticed it in other Hoplonemertea. It 

 is figured on PI. X. fig. 1, ne, and consists in the fact of a peripheral stem, which 

 has taken its outward course away from the longitudinal nerve-trunk, and which 

 has penetrated amongst the pennate fasciculi of longitudinal muscle-fibres of the layer 

 a, spreading out in a plane parallel to that of the body-surface, and thus forming a kind 

 of local plexus between the muscular layers a and /3. It must for the present remain an 

 open question whether this arrangement, which can be noticed in different regions of the 

 same section, and which in no section was absent, must be regarded as a primitive 

 feature connected with the plexiform arrangement which must have obtained in the 

 ancestral forms of the Hoplonemertea, or whether it is merely a special adaptation, 

 having arisen in certain Hoplonemertea, and being in some way subservient to the 

 innervation of the muscular investment or the integument. At all events it is a peculiar 

 arrangement, and, as such, deserves special mention. 



How intricately and yet how regularly the peripheral nerve-system of the Hoplo- 

 nemertea may be said to be distributed can also be gathered from Moseley's figure of 

 Pelagonemertes, which we have copied on PL I. fig. 23, where the peripheral nerves 

 are seen to spring, two at a time, from the lateral trunks, which here, too, are united 

 posteriorly by a commissure above the anus. 



(zool. CHALL. EXP. — part liv. — 1887.) Hhh 12 



