EELATION OF THE NEMERTEA TO THE VERTEBRATA. 629 



the phenomenon which shows us the musculature of the right 

 and left half of the body, developing quite independently in 

 the Chordata. It is easy 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 meta- 

 mery became gradually established, (2) the dorso-median me- 

 dullary 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, offer 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 Amphioxus were the only Vertebrate known, we should, 

 recognising the phylogenetic 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 



