CHAPTER X 



PODAXONIA 



Classification adopted — 



Podaxonia (Gephyrea nuda) | p^ . , 



In the old group of the Gephyrea, which used to be regarded as a 

 subdivision of the Annelida, there were included several families or 

 sub-orders of such diverse structure that it has been recently 

 customary to separate them entirely from each other and to regard 

 them as belonging to quite distinct phyla. Of these families one, 

 the Echiuroidea, is undoubtedly to be regarded as a modified group 

 of Polychaeta ; and about another, the Priapuloidea, nothing can be 

 said until the development has been worked out, and we know more 

 about the adult anatomy of its members. A third group, the 

 Sipunculoidea, constituting the Gephyrea nuda, agrees with Annelida 

 and MoUusca in possessing a Trochophore larva, and hence must be 

 regarded as descendants of the same Ctenophore-like ancestor, from 

 which, as we have seen reason to believe, these two phyla originated. 

 They differ, however, from both Annelida and MoUusca in the fact that 

 the principal extension of the body takes place in a direction almost 

 at right angles to the line joining mouth and anus, and further- 

 more in a ventral direction. Hence the name Podaxonia, coined for 

 them by Eay Lankester (1890) with his customary insight. 



It is probable that the group of Ectoproct Polyzoa is allied to 

 the Podaxonia, but the full proof of that is a matter to be settled by 

 future investigation. The group of the Phoronidea, however, con- 

 stituting the old division of Gephyrea tubicola, is almost certainly 

 closely allied to the Podaxonia, of which it will be considered a sub- 

 division. Evidence in favour of this view will be offered in this 

 chapter. 



PHASCOLOSOMA 



The genus PJiascolosoma has representative species on both sides 

 of the Atlantic. The cell-lineage and larval development of the 



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