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



stems leave the brain in corresponding regions of the lower brain-lobe, i.e., along the surface 

 turned towards the proboscidian sheath, and run in the direction of the oesophageal 

 epithelium. This secondary innervation, though different in morphological aspects, is 

 more or less homologous with the facts above disclosed in the case of Cerebratulus corru- 

 gatus. That the vagus proper — the massive and thick stem — is here turned forwards 

 may be a consequence of the change in the situation of the mouth, which in the Hojdo- 

 nemertea is no longer behind the brain, but in front of it. This gradual change of 

 position may very possibly have drawn the vagus-stem with it. In concluding our 

 remarks upon the brain I have only to add that the well-known difference between 

 Schizonemertea and Hoplonemertea with respect to the connection between anterior and 

 posterior brain-lobes (side organs) also obtains in the Challenger specimens. The latter 

 are connected in Drepanophorus and Amphiporus with the brain by one or more fibrous 

 commissures. 



Another difference several times observed between the fibrous brain-tissue of these 

 two Hoplonemertean genera on the one hand, and Cerebratulus, Eupolia, &c, on the 

 other, is a marked increase in compactness of the fibres, so much so that the fibrous 

 character of the central portions of the brain has often more the aspect of Ley dig's 

 " Punktsubstanz," and even shows a still more delicate and more compact texture by 

 the appearance in this " Punktsubstanz " of regular patches with very faint outlines, 

 which apparently are still more compact regions of this tissue. 



. The longitudinal nerve-stems, which are the posterior continuations of the lower 

 brain-lobes, hardly need any special mention. It must only be insisted upon that in 

 them, as well as in the brain-lobes, there is no absolute distinction between the cellular 

 envelope and the fibrous core, but that inside this core nuclei are invariably scattered, 

 which bear testimony to the absence of any such definite boundary. Still, there is 

 generally a homogeneous and very thin layer between the cellular coating and the fibrillar 

 core, a kind of membranous neuroglia, through which the fine processes of nerve-cells may 

 be seen to take their course in groups, which then become lost amongst the fibres of the 

 core. Then, again, certain favourable sections (PI. XII. fig. 2) very distinctly show the 

 course of nerve-fibres inside this fibrous core that are not longitudinal, and thus puncti- 

 form in transverse sections, but that are interwoven at right angles with the latter and 

 continue their way into the nerve-plexus. The transparent sheath of the fibrous core 

 of the nerve-stem is more distinct in Cerebratulus than in either Pakeonernertea or 

 Hoplonemertea. It is rarely encountered in the brain, where fibrous and nervous elements 

 are more intimately interwoven (cf. PI. VI. figs. 4-8; PI. XII. figs. 7, 8; PI. XIV. figs. 7, 8). 

 Outside of the stems there is another accumulation of homogeneous connective 

 tissue arranged as a protecting envelope round the nerve-stems. This is much more 

 conspicuous in the brain-lobes, and more so in the Hoplonemertea (PI. XII. fig. 5) than 

 in the Schizonemertea (PI. XII. fig. 2 ; PI. XIV. fig. 2). Still in the latter it is far more 



