46 



relationship with the Chaetopoda, but there is a want of 

 agreement between the groups in some points which makes 

 the matter doubtful, and therefore it is safer at present not to 

 regard the Brachiopods as degraded Chaetopoda, but as 

 derived from a lower point in the vermean series. The exact 

 position of that point of origin is very doubtful ; probably it 

 lay above the primitive Gephyrea, but below the origin of the 

 Kotifera. The present Brachiopods must be regarded as 

 having degenerated in accordance with their sessile mode 

 of life. 



The Polyzoa are also degenerate forms belonging in all 

 probability to this division of the Vermes. The larvae are 

 free-swimming ciliated forms, which may be compared with 

 the Trochosphere stage found in the development of so many 

 Vermes. Balfour* regards them as Trochospheres, which 

 became fixed in the adult by the extremity of the prse-oral 

 lobe. He also shows that there is reason to consider the 

 Polyzoa as exhibiting alternation of generations. The ovum 

 develops into a free-swimming form (the so-called larva), 

 which never becomes sexual, but produces by budding the 

 attached form (the adult Polyzoon), which develops repro- 

 ductive organs. From the fact that Cyphonautes, the larva of 

 Membranipora, an ectoproctous Polyzoon, is itself entoproct- 

 ous, it is probable that the Entoprocta (Pedicellina) are more 

 primitive than the Ectoprocta. Both groups are, however, 

 degenerate and considerably modified. 



The Chaetognatha (Sagitta) are a small group with 

 obscure affinities, which are best placed in this part of the 

 vermean series close to the origin of the Arthropoda. 

 Possibly they may be more closely related to the Nematelmia 

 than is shown by the table. 



The Rotifera are certainly degraded, but what they have 

 been derived from is somewhat doubtful. Pedalion appears 



* Comp. EmbryoL, v. i, p. 255. 



