REPORT ON THE NEMERTEA. 131 



unknown invertebrate ancestors, and I have insisted not only on tlie possibility of the 

 homology between the Nemertean proboscis and the hypophysis cerebri of the Verte- 

 brates, but I have, even earlier still, attempted to show that the nerve-system of these 

 two groups might be considered in a common light, as was first indicated by Harting 

 in his Leerboek van de Grondbeginselen der Dierkunde of the year 1874. Further 

 reference to the hypothesis here alluded to is found in Balfour's Monograph on the 

 Elasmobranch Fishes (pp. 170-172), in my own publications (IX, X), and in Balfour's 

 Comparative Embryology^ (vol. ii. p. 258). I will not here enter upon this hypothesis 

 more fully, but will briefly state that it attempted to consider the central nervous system 

 of the Vertebrates as a possible median coalescence of two nerve-trunks, that were lateral 

 in the primitive ancestors of the Vertebrates, in the same way as the coalesced ventral 

 nerve-cord (Bauchmark) of Annelids and Arthropods may be considered with Gegenbaur 

 as having arisen out of a double lateral trunk, which in certain, still more highly difiieren- 

 tiated forms have fused ventro-medially. 



A strong argument against the first-mentioned hypothesis is the fact tliat the spinal 

 cord ontogenetically always makes its appearance as a median unpaired plate or thicken- 

 ing, a very faint trace of a possible double origin of this jilate being hitherto only 

 observable in one species of Amphibia, Triton tseniatus ; whereas in all other vertebrates, 

 Amphioxus 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 hy]3othesis, because I will attempt to show 

 that we can establish phylogenetic comparisons 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, some- 

 times of comparatively large size, has now been detected, since even in the Hoplo- 

 nemertea, 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 



1 It may here he remarked that Balfour has omitted to mention that Harting was the first to bring forward this 

 hypothesis : it is well to be reminded of this when Beard, Bateson, and others similarly ignore this claim to priority of 

 my venerated predecessor. 



