362 A. A. W. HUERECHT 



conversion of a hollow proboscidian sheath into a solid noto- 

 chord, the more so as functionally the proboscidian sheath in 

 Nemertines may already be looked upon as an axis, around 

 which the other organs symmetrically arrange themselves as 

 they do around the notochord in Vertebrates. It must at the 

 same time be borne in mind that the muscular coating in this pos- 

 terior portion is found to be considerably reduced and replaced 

 by a more or less homogeneous and comparatively thin sheath. 



Having thus far considered the arguments which are at pre- 

 sent available for insisting upon the homology between proboscis 

 and hypophysis, on the one hand, and between proboscidian 

 sheath and notochord on the other, it now remains to inquire 

 whether there are points in the anatomy of Nemertines, beyond 

 those just now exposed, which either corroborate or weaken the 

 evidence hitherto advanced in favour of the suggestion that the 

 Nemertines, more than any other known group of Invertebrates, 

 resemble the ancestors of the Protochordata. 



I need hardly insist upon the fact that I do not advocate any 

 direct relation between existing Nemertines and existing Verte- 

 brates ; my argument goes no further than the attempt to show 

 that the general plan of structure of a Nemertine is more in 

 accordance with that of a vertebrate animal than is, for ex- 

 ample, that of the Archiannelida, and that the link connecting 

 Coelenterate ancestors with Vertebrate descendants has most 

 probably comprised forms in which two lateral nerve-cords 

 were present, ultimately coalescing dorsally, and in which an 

 epiblastic proboscis served for purposes which have either been 

 oiven up or have been replaced by others when the animals 

 gradually exchanged the Platyelminth for the Chordate type. 



Simultaneously with this passage from the Cwlenterate 

 type to the Chordate the highly important processes must have 

 been gone through which lead to the formation of a body-cavity, 

 separate from the archenteron with which, as embryology teaches 

 us, certain diverticula are originally in open communication, 

 ultimately becoming constricted off and developing into the 

 splanchnic and somatic layers, which have the body-cavity 

 between them. 



