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



before Bateson drew renewed attention to the numerous points of agreement between 

 Balanoglossus and the Chordata, M'Intosh had done the same for Balanoglossus and the 

 Nemertea, a sepai-ate paragraph of his monograph (XIX) being devoted to the discussion 

 of these homologies. 



Sedgwick [loc. cit.) holds the unsegmented worms to be wholly " negligeablc 

 quantities," at any rate superfluous links in the chain that connects the Chordata with 

 the antecedent Diploblastic stages. In my idea both these authors, valuable as certain 

 of their suggestions are, have not been thoroughly aware of the necessity that, in all 

 discussions on the origin of metameric segmentation, we must attempt to grasp at data 

 that give a clue to the possible action of natural selection in the gradual evolution of 

 metamery. This clue appears to me to be far more distinctly contained in the views here 

 advocated than in the other h}^otheses. 



It may further be remarked, now that we have once more alluded to Bateson's 

 phylogeny of the Chordata, that even this naturalist does not feel justified in wholly 

 rejecting the Nemertea from the Vertebrate pedigree. Whilst in the text of his 

 article {loc. cit., p. 566) he does seem to prefer this negative alternative ; still, 

 in the subjoined diagram of the general relationships of Urochorda, Hemichorda, 

 Cephalochorda, and Vertebrata, the Nemertea are introduced — with a point of 

 interrogation, however — as a side branch lower down on the common parent stock. 

 Now, this being concordant with my own views of the Chordate phylogeny, — 

 the point of interrogation excepted, — it is necessary to incpiire why there is this 

 discrepancy between Bateson's speculations in the body of his treatise and the 

 hypothetical pedigree at the end of it. It appears to me that this is due to his 

 hesitation (loc. cit., p. 555) in accepting the views hitherto entertained and advocated 

 by myself as to the phylogeuetic connection between the Nemertean and the Vertebrate 

 nervous system. For this hesitation Bateson has good reasons, and while I appreciate 

 the soundness of them, I hope in the remainder of this chapter to remove the reluctance 

 of him and others to accept the phylogeuetic significance of the Nemertea, thanks to 

 new light that may be thrown on the evolution of the central nervous system of 

 the Chordata by the observations above recorded on the nervous system of the Challenger 

 Nemertea. 



It is to these speculations on the nervous system that we now have to turn our 

 attention. 



As wiU be seen from the terminology introduced in the paragraph on the nervous 

 system (p. 76), and as it is now time more fully to develop, I am inclined to attach 

 considerable morphological importance to the arrangement of the diff"erent constituent 

 parts of the nervous system in the Nemertea. In former publications (X, XIa) I have 

 repeatedly insisted on the significance of certain points in the anatomy of the Nemertea, 

 when considering the general question of the relationship of the Chordata to their 



