364 \< T1NOTROCHA. 



While these changes have been taking place the prae-oral 

 lobe has become much contracted, and partly withdrawn into 

 the stomodajum. At the same time the arms have become bent 

 forward, so as to form a ring round the mouth. Their bases 

 become much thickened. The metamorphosis is completed by 

 the entire withdrawal of the prae-oral lobe within the oesophagus, 

 and by the casting off of the ends of the arms, their bases 

 remaining as the circumoral ring of tentacles, which form 

 however a lophophore rather than a complete ring. The 

 perianal ring of cilia is also thrown off, and the anal process 

 withdrawn into the body of the young Phoronis. There are now 

 three longitudinal vascular trunks, united anteriorly by a circular 

 vessel which is prolonged into the tentacles. 



General Considerations. 



The development of Phoronis is so different from that of the 

 other Gephyrea that further investigations are required to shew 

 whether Phoronis is a true Gephyrean. Apart from its peculiar 

 metamorphosis Actinotrocha is a very interesting larval form, in 

 that it is without a prae-oral ciliated ring, and that the tentacles 

 of the adult are derived from a true post-oral ring, prolonged 

 into arm-like processes. 



The other Gephyrea present in their development an obvious 

 similarity to the normal Chaetopoda, but their development stops 

 short of that of the Chaetopoda, in that they are clearly without 

 any indications of a true segmentation. In the face of what is 

 known of their development it is hardly credible that they can 

 represent a degenerate Chaetopod phylum in which segmentation 

 has become lost. Further than this the Gephyrea armata seem 

 in one respect to be a very primitive type in that they retain 

 through life a well-developed pra-oral lobe, which constitutes 

 their proboscis. In almost all other forms, except Balanoglossus, 

 the larval prae-oral lobe becomes reduced to a relatively in- 

 significant anterior part of the head. 



BIBLIOGRAPHY. 

 Gephyrea nuda. 



(366) A. Kowalevsky. Sitz. d. zool. Abth. d. III. Vcrsam. russ. Naturj. 

 (Thalasscma). Zeit.f. wiss. Zool. Vol. xxn. 1872, p. 284. 



