EELATION OF THE NEMERTEA TO THE VERTEBRATA. 623 



deeper strata of the integument. The only difference between 

 the medio-dorsal medullary nerve in this species and the lateral 

 nerves with their anterior enlargements (the brain-lobes) is its 

 position and its greater tenuity (PL XLII, fig. 1), which, 

 however^ does not prevent its being very clearly observable in 

 every transverse section. Its connection with the brain-com- 

 missure was already described and figured by me (loc. cit., p. 25, 

 pi. iii, fig. 31). It must, however, be remarked that in these 

 most primitive Palseonemertea the anterior dorsal brain-com- 

 missure is less significant than in the Schizonemertea, and 

 hardly anything else than the foremost of those numerous 

 transverse metameric tracts in the plexus {dvr, PL XLII, 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. 

 Posteriorly the medullary nerve can be followed down to the 

 hindmost extremity of the body. In Eupolia and the Schizo- 

 nemertea the arrangement 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 type as 

 represented in the diagrammatic fig. 1 on PL XLII. 



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 con- 

 trary, 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 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 



