GEPHYREA. 363 



It probably belongs to a different species. The body is uni- 

 formly ciliated (fig. 163 A). There is a large contractile prae-oral 

 lobe, and the body ends behind in two processes. The mouth 

 (m) is ventral, and the anus (an} dorsal, and not terminal as in 

 Kowalevsky's larva. 



The alimentary tract is divided into stomodaeum, stomach 

 and intestine. The two processes at the hind end of the body 

 are the rudiments of the first-formed pair of the arms which are 

 so characteristic of the fully developed Actinotrocha. A second 

 pair of arms next become established on the dorsal side of the 

 previously existing pair, and the region where the anus is placed 

 grows out as a special process. New pairs of arms continue to 

 be formed in succession dorsalwards and forwards, and soon 

 constitute a complete oblique post-oral ring (fig. 163 B). They are 

 covered by long cilia. Round the anal process a very conspicuous 

 ciliated ring also becomes established. 



At the period when five pairs of arms are present a delicate membrane 

 becomes visible on the ventral side of the intestine which joins the somatic 

 mesoblast anteriorly. This membrane is the rudiment of the future ventral 

 vessel. The somatic mesoblast is present even before this period as a 

 delicate layer of circular muscular fibres. 



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 alimen- 

 tary canal passes into it, and at the same time the body of the 

 larva becomes violently contracted. By the time the evagi- 

 nation is completed it forms (fig. 162 E) a long conical body, 

 containing the greater part of the alimentary tract, and consti- 

 tuting the body of the young Phoronis. The original anal process 

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



