350 A. A. W. HUBEECHT 



shape with Polygordius, and that the only postulate which 

 this assumption necessarily implies, is the old idea of 

 GeofFroy St. Ililaire that the ventral side of Annelids and Arthro- 

 pods is homologous with the dorsal side of the Vertebrata. 



These naturalists explain the difference in situation of the 

 moulh and oesophagus, with respect to the cerebral ganglion, 

 by divers subtle hypotheses, which, however, generally dis- 

 agree with each other. Their views are nevertheless rapidly 

 gaining ground, although the school of Gegenbaur and 

 Haeckel has never been reconciled to them. Gegenbaur looks 

 upon two lateral cords such as are present in Nemertines 

 as a very primitive arrangement, from whence might at 

 any rate be derived tlie ventral nerve-cord of Annelids and 

 Arthropods. Harting (' Leerboek der Dierkunde,' 1874) was 

 inclined to accept the possibility of a similar dorsal coales- 

 cence out of which a spinal cord might take origin. Balfour 

 and myself felt strongly inclined to choose this side in the 

 contest — he in once more tracing the outlines of a similar 

 explanation {' Development of Elasmobranch Fishes '), I 

 in recapitulating the facts, such as they had made them- 

 selves known to me in the organisation of certain Nemer- 

 tines, in which, indeed, a tendency towards approxima- 

 tion of the lateral cords on the dorsal side was unmis- 

 takable ('Verb, der Kon. Akad, van Wetenschappen,' Am- 

 sterdam, 1880). When Balfour, in the second volume of his 

 ' Comparative Embryology,' made himself a definite advocate 

 of this view in opposition to the combatants for the Annelidan 

 affinities, it may be safely inferred that many of the younger 

 naturalists paused to reconsider the claims of both hypotheses. 



The great difficulty which is encountered in any attempt to 

 point out a definite group amongst invertebrates most closely 

 related to the primitive Vertebrata is the total absence of 

 anything resembling so important and so early-formed an 

 organ as the Vertebrate Chorda dorsalis. Attempts to find 

 anything like it amongst the Annelids, even amongst Polygor- 

 dius and its archaic allies have proved either futile or barren. 



I will at present attempt to point out in what group of 



