GEPUYREA. 301 



When six pairs of arms have become formed an involution (fig. 

 163 C, iv) appears on the ventral side, immediately behind the ring 

 of arms. This involution consists both of the epiblast and somatic 

 mesoblast. It grows inwards towards the intestine, and, increasing 

 greatly in length, becomes at the same time much folded. 



When it has reached its full development the critical period of 

 the metamorphosis of Actinotrocha into Phoronis is reached, and is 

 completed in about a quarter of an hour. The ventral involution 

 becomes evoluted (fig. 163 D), just as one might turn out the finger 

 of a glove which had been pulled inwards. When the involution has 

 been to a certain extent everted, the alimentary canal passes into it, and 

 at the same time the body of the larva becomes violently contracted. 

 By the time the evagination is completed it forms (fig. 162 E) a long 

 conical body, containing the greater part of the alimentary tract, and 

 constituting ilie body of the young Phoronis. The original anal process 

 remains on the dorsal side as a small papilla (fig. 162 E, an}. 



While these changes have been taking place the prse-oral lobe has 

 become much contracted, and partly withdrawn into the stomodseum. 

 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 prse- 

 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 pro- 

 cess withdrawn into the body of the young Phoronis.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 Chtetopoda, but their development stops 

 short of that of the Chsetopoda, 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 Chsetopod 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 

 pros-oral lobe, which constitutes their proboscis. In almost all other 

 forms, except Balanoglossus, the larval pra3-oral lobe becomes reduced 

 to a relatively insignificant anterior part of the head. 



