406 INVERTEBRATA CHAP, xi 



The fixing organ of the Cyphonautes larva is the eversible 

 sucker, and this is a modification of the ventral surface between 

 mouth and anus, and corresponding to the region which, in Phoronis, 

 is everted to form the worm-like body of the adult, and the region 

 which grows into the long worm-like body of the Sipunculoidea. 

 This latter group, as we have seen, are certainly, and Phoronis is 

 probably, derived from a Trochophore stock. 



We regard, therefore, the Polyzoa Ectoprocta as a degenerate 

 group, allied to Phoronis and the Sipunculoidea, which have become 

 fixed by the ventral surface and have learned how to bud. It 

 follows that Entoprocta and Ectoprocta, though both derived from 

 the Trochophore stock, in common with all Annelida and Mollusca, 

 have acquired a fixed life in different ways and at different periods ; 

 and are not descended from a common fixed Polyzoan ancestor, but 

 are two separate offshoots from the Trochophoran stock. It follows 

 also that Polyzoa Ectoprocta alone can be regarded as having affinities 

 with Podaxonia, and that Polyzoa Entoprocta cannot be included in 

 this group, as defined by Lankester (1890). 



The fact that in the formation of buds the value of the germinal 

 layer is quite altered, and that tentacles and atrial wall, along with 

 the alimentary canal, arise from a common rudiment, will be con- 

 sidered when the similar phenomena amongst Tunicata (Urochorda) 

 are discussed. 



LITERATURE CONSULTED 



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