RELATION OF THE NEMERTEA TO THE VERTEBRATA. 621 



and the Cyclostomata not excepted, the unpaired origin is most 

 evident. The bilateral symmetry of the full-grown brain and 

 spinal cord is a much later feature, and can hardly be regarded 

 as the expression of a primary coalescence of two separate 

 halves to form a median whole. 



I am the more inclined to abandon this hypothesis because 

 I will attempt to show that we can establish phylogenetic com- 

 parisons between the Chordate and the Nemertean nervous 

 system on a much more simple basis ; comparisons which at 

 the same time cover a far more extensive ground than did 

 those of Harting, Balfour, and myself, which I have just 

 alluded to. 



• Since in the nervous plexus of all the Nemertea a median 

 longitudinal tract, sometimes of comparatively large size, has 

 now been detected, since even in the Hoplonemertea, where 

 the plexus has disappeared, the same medio-dorsal nerve-tract 

 has in most cases been preserved, and, finally, since from this 

 dorso-median stem metameric and paired nerve-tracks may be 

 seen to emerge in Palseonemertea and Schizonemertea, we 

 must inquire in how far the direct comparison of this medio- 

 dorsal nerve-stem with a primitive spinal cord may be said to 

 hold good. 



In order to do this we must first consider the relation of 

 this stem, to which we have given the name of medullary nerve 

 or medulla, to the rest of the nervous system, more especially 

 the brain-lobes.^ In a former publication,^ where the medullary 

 nerve was for the first time noticed and described as the pro- 

 boscidian-sheath-nerve, I traced its origin to the dorsal com- 

 missure between the two lateral halves of the brain (loc. cit., 

 pi. i, fig. 1). Thanks to certain very favorable specimens in 

 the Challenger collection, I have now been able to add new 

 data to this statement. Sections through the brain of Cere- 

 bratulus macroren, Cerebratulus corrugatus, and 



^ In the course of these considerations a certain amount of repetition of 

 facts already noticed in the paragraph on the nervous system cannot well be 

 avoided. 



* Verhand. Kon. Akad. v. Wetensch., Amsterdam, vol. xx, 1880. 



