620 A. A. W. HUBREOHT. 



have repeatedly insisted on the significance of certain points in 

 the anatomy of the Nemertea, when considering the general 

 question of the relationship of the Chordata to their unknown 

 invertebrate ancestors^ and I have insisted not only on the 

 possibility of the homology between the Nemertean proboscis 

 and the hypophysis cerebri of the Vertebrates, 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 Grond- 

 beginselen der Dierkunde,' of the year 1874. Further reference 

 to the hypothesis here alluded to is found in Balfour's Mono- 

 graph on the Elasmobranch Fishes (pp. 170 — 172), in my own 

 publications^ — , 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 differentiated, forms have fused ventro-medially. 



A strong argument against the first-mentioned hypothesis 

 is the fact that the spinal cord ontogenetically always makes its 

 appearance as a median unpaired plate or thickening, a very 

 faint trace of a possible double origin of this plate being 

 hitherto only observable in one species of Amphibia, Triton 

 tseniatus; whereas in all other Vertebrates, Amphioxus 



' " Zur Anatomie und Physiologie des Nervensystems der Nemertinen," 

 ' Verhandel. van de Koninkl. Akad. van Wetenscbappen/ Amsterdam, 1880, 

 vol. XX. " The Peripheral Nervous System of the Palaeo- and the Schizo- 

 nemertea, one of the layers of the body-wall," * Quart. Journ. Mior. Sci.,' vol. 

 XX, 1880. 



'•' It may here be 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. 



