REPORT ON THE NEMERTEA. 105 



While fully recognising the importance of Graff's observations for our own interpretation 

 of the Nemertean proboscis, and the genetic relation of this organ to that of the Rhab- 

 docceles (not direct but collateral), I must as emphatically reject the proposed derivation 

 of the proboscidian sheath advocated by Salensky. 



We must, indeed, represent to ourselves the gradual evolution of the proboscis as 

 that of an epiblastic organ reaching further and further inwards in successive genera- 

 tions, and strengthened and completed by a mesoblastic musculature ; and outside of this 

 the free and independent development out of other mesoblastic elements (primarOy 

 belonging to the body-wall) of the sheath. It has been abeady noticed elsewhere (XIV, 

 XV), that if these mesoblastic structures could be traced down to amoeboid mesoblast 

 cells derived in loco out of the subjacent hypoblast, an ontogenetic homology between 

 the tissues constituting the proboscidian sheath and those forming the notochord of 

 Vertebrates would be established. 



Returning to the proboscidian sheath of the Schizonemertea, we find it to consist of 

 an outer layer of circular fibres and an inner one of longitudinal (PL X. fig. 8, mPrs; 

 PI. XV. fig. 1). The former sometimes, when the sheath is thick and contracted, shows 

 a wavy line. Radial fibres, piercing the two fibrous layers, insert themselves against the 

 inner epithelium, which covers the whole inner surface, looking towards the cavity of the 

 proboscidian sheath. Between this epithelium and the muscular layers there is a broad 

 band of transparent basement tissue (PI. XV. fig. 1, b) following the numerous longitu- 

 dinal folds of the epithelium just mentioned. These folds disappear when the proboscidian 

 sheath is in distension (PL X. fig. 8), a phase that may repeatedly be noticed, even 

 without any extrusion of the proboscis, e.g., as a consequence of complicated coilings of 

 the proboscis inside its sheath. It is easily understood that during such distension the 

 thickness of the subepithelial homogeneous basement layer and of the muscular layers is 

 considerably reduced. A maximum degree of distension is figured on PL X. fig. 9, where 

 the epithelium was no longer separately visible, and even the (Esophageal epithelium has 

 been flattened out, together with the proboscidian-sheath wall. 



As will be seen from PL XV. fig. 1, we find outside of the outer circular layer of the 

 sheath the gelatinous body-parenchyma, a thin layer of this even separating the probos- 

 cidian sheath from the longitudinal muscular layer a, in the midst of which we notice the 

 true proboscidian-sheath-nerve (pr.sn). In addition, I think it is not unimportant to 

 remark, that just below this layer of longitudinal fibres, there are strands of circular fibres 

 which do not apparently belong to the proboscidian sheath, and which, after having been 

 closely applied against the dorsal musculature in the middle line of the back, radiate 

 amongst the parenchyma and the intestinal caeca. It is these fibres (and perhaps in 

 addition to them the circular layer of the sheath itself) which may possibly be looked 

 upon as representative of the layer 8 in the Carinellidae {cf. PL XL), and which there takes 

 such a conspicuous part in the dorsal delimitation of the proboscidian sheath. 



(ZOOL. CHALL. EXP. PART uv.— 1887.) Hhh 14 



