Phylogeny of Animals 539 



tudinal system. The simple or rhabdocoel Turbellaria how- 

 ever soon spht up into a series that not only retained the dorso- 

 lateral nerves; these became approximated dorsally and in- 

 creasingly strong. 



The latter condition seen in the Nemertinea we have already 

 emphasized as a perfect transition phase, that conducts from 

 the Rotifera through the rhabdocoel turbellarians upward 

 to the simplest chordates or vertebrates. So when the flexible 

 and tactile protrusible oral area of some rotifers became the 

 dorsal protrusible proboscis of some turbellarians, and later 

 became modified into the dorso-cephalic proboscis of early 

 nemerteans, increasingly abundant, varied, and powerful nerve 

 stimuli passed centripetally to it, and centrifugally from it. 

 Such action and reaction stimulated the dorsal and the lateral 

 at the expense of the ventral nerve cords. When therefore 

 the steadily strengthened proboscis-sheath became a strength- 

 ening tube or rudimentary notochord, as claimed by Hubrecht, 

 the attenuate dorsal nerve cord became the precursor of Reiss- 

 ner's fiber, while the laterals became the dominant trans- 

 mitters of stimuli, and later by dorsal approximation and 

 union formed the vertebrate spinal cord. 



But during the past half century much attention has been 

 given to three aggregates of marine organism, that show more 

 or less markedly chordate characters. These are the Hemi- 

 chordata or Balanoglossus series, the Urochordata or ascidian 

 series, and the Cephalochordata or lancelet series. The first 

 of these seems for many reasons to be a greatly modified marine 

 derivative from marine nemertean ancestry, in which the 

 larva has at times become distinctly pelagic, and the adult 

 animal has undergone highly divergent modification from 

 an advanced and specialized nemertean ancestor. In line 

 with such a view, we would regard the proboscis as the j)er- 

 manently unfliped or evaginated proboscis of a nemertean, 

 into the interior of which the nemertean proboscis-sheath 

 has been pushed forward as a condensed and collapsed noto- 

 chord, and is surrounded internally by the masses of muscle 

 that in earlier history gave it its flexible character. The basal 



