REPOET ON THE NEMERTEA. 133 



however, be remarked that in these most primitive PalfBonemertea, the anterior dorsal 

 brain-commissure is less significant than in the Schizonemertea, and hardly anythino- else 

 than the foremost of those numerous transverse metameric tracts in the plexus {dvi\ 

 PI. XVI fig. 1) which connect the lateral stems with the medullary nerve (dorsally) and 

 with each other (ventrally). 



These important metameric nerve-pairs are most distinctly observed in Carinella. 

 Here, as in the Schizonemertea, the medullary nerve is also continued forwards in front 

 of the brain thickenings. This continuation sometimes shows a short bend just on 

 the level of the commissure, so that both the medullary nerve and its anterior 

 continuation may be seen in one section. This explains at the same time the arrange- 

 ment traced on PL XII. fig. 8. Posteriorly the medullary nerve can be followed down to 

 the hindmost extremity of the body. In EwpoUa and the Schizonemertea the arrange- 

 ment remains the same, the metamery of the transverse stems is perhaps more clearly 

 expressed, the whole plexus and the longitudinal stems are no longer in the integument, 

 but between the muscular layers. Still the whole of the nervous system also answers to 

 the general ty^ie as represented in the diagrammatic fig. 1 on PI. XVI. 



We have now seen enough of it to understand that a comparison with the central 

 apparatus of the Vertebrate nervous system cannot indeed be called a strained comparison. 

 On the contrary, the comparison is much less artificial than was the one which Balfour 

 was inclined to adopt, and which, as noted above, rendered necessary the acceptance of 

 the phylogenetic development of the Vertebrate medulla out of a double cord. 



And so I do not hesitate to proclaim the medullary nerve of the Nemertea to be a 

 very important link in the phylogenetic chain, of which the Vertebrate spinal cord is the 

 outcome. Like the Nemertean medulla, the Vertebrate spinal cord is median, unpaired, 

 and composed of nerve-cells and nerve-fibres ; like the Nemertean medulla, it is a thicken- 

 ing in a nervous plexus, originally wholly epiblastic, of which, among Vertebrates, the 

 Amphibian embryos ofi'er such a striking example. This instructive and suggestive case 

 was known to Eemak and Strieker (as the " Nervenschicht" of the frog embryo), it was 

 more carefully studied and elaborately described by Goette (his " Grundschicht" of the 

 epiblast, in his Entwickelungsgeschichte der Unke), and it has been again recently 

 brought into the foreground by Baldwin Spencer, in his latest paper on the subject.* 

 The latter author compares the Amphibian plexus with that of Palseonemertea and Schizo- 

 nemertea {loe. cit., p. 134), as had already been done before him by my friend Professor 

 Pvay Lankester, with whose suggestion I at that time (1880) did not yet venture fully 

 to associate myself. 



The numerous data that have since been accumulated for a direct comparison of 

 Nemertea with lower Vertebrates appear, however, now to fully justify that comparison 



> Baldwin Spencer, Some notes on the early Development of Rana temporaria, Quart. Journ. Micr. Sci., vol. xsv. 

 Snppl., p. 123, 1885. 



