300 



ACTINOTROCHA. 



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 (rig. 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 

 raesoblast 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. 



Fin. 163. A SKRIES OF STAGES IN THE DEVELOPMENT OF PHORONIS FROM 

 ACTINOTROCHA. (After Metschnikoff.) 



A. Young larva. 



B. Larva after the formation of post-oral ring of tentacles. 



C. Larva with commencing invagination to form the body of Phoronis. 

 P. Invagination partially everted. 



E. Invagination completely everted. 



m. mouth; an. anus; i<\ invagiuation to form the body of Phoronis. 



