EEPORT ON THE NEIIERTEA. 137 



pendently in the Chordata. It is easily intelligible how, as this phenomenon gradually 

 becomes more and more marked, no more ventral connecting fibres across the non- 

 muscular region were required for the innervation of the musculature of the right and 

 left half of the body. 



The process by which the transverse nerve-tract, with radial nerve-fibres leaving it at 

 short intervals, both centripetally and centrifugally gradually assumed the form of a 

 nerve-stem with a dorsal and a ventral branch, such as we find in the spinal nerves, must 

 have gone on 'pari passu with those numerous other changes, which we cannot as yet 

 fully trace, but which must have occurred when (1) the muscular metamery became 

 gradually established, (2) the dorso-median medullary tract became so preponderant 

 that an increase in mass, with economy of bulk, was only to be obtained by a process of 

 folding-in already discussed above, and (3) the attachment of the spinal nerves 

 (transverse tracts) to the medulla was modified in consequence of this process. 



None of these phenomena, however, oflfer anything that is in any way inconsistent 

 with, or opposed to, the general theory here developed. 



We have now sufficiently insisted on the chief point of comparison here proposed, 

 viz., that between the Nemertean medullary nerve and its metameric transverse nerve- 

 cords, and the Vertebrate cerebro-spinal axis and spinal nerves. 



If Amjihioxus were the only Vertebrate known, we should, recognising the phylo- 

 genetie importance of the plexiform arrangement still met with in the adult of that 

 species, admit that, as far as we know at present, the primary lateral nerves with their 

 anterior swellings of the Vermian ancestors had disappeared in the same measure as the 

 dorso-median spinal cord had come more and more into the foreground. 



But our consideration of other Vertebrates leads us to the conclusion that, when 

 once the general homology between the two nervous systems is admitted, there may 

 perhaps be secondary points in regard to which the comparison can be further extended. 

 And it must be recognised, that if we should also succeed in rendering more or less 

 probable a comparison in secondary details, this might again be favourably interpreted 

 for the primary and more important part of the hypothesis. 



The search after these secondary points of agreement was instituted by me, when the 

 question above alluded to presented itself, viz., if any remnant could be traced of the 

 central nervous system of Nemertea-hke ancestors, i.e., of the brain-lobes and lateral 

 stems, in those Vertebrate descendants in which the medio-dorsal tract had become so 

 preponderant as to give rise to the unpaired medulla and brain. 



It is clear that if it shall be possible to trace any such remnants, and to render their 

 homology with the Nemertean central nervous system probable, they will have to be 

 sought for — (o) in the head, as lateral more or less independent nerve-centra, innervating 

 sense-organs of the integument, and passing posteriorly into parallel longitudinal stems ; 

 or (h) in the trunk, as longitudinal nerve-stems, in which the central character should 



(ZOOL. CHALL. EXP. PART LIV. — 1887.) Hhh 18 



