896 ARTHUR T. MASTER]\]AN. 



invagination of an epiblastic portion carrying the actual 

 mesoblastic pores into the interior of the mesoccele. 



The metacoeles appear to have fused, and to lie as a shallow 

 (from side to side) coelomic sac dorsal to the stomach and 

 intestine. They are surrounded in front and on either side 

 by the mesocoeles, and their walls form with them a con- 

 spicuous pair of dorso-lateral mesenteries (fig. 48), and a 

 median transverse mesentery (fig. 47). 



Fig. 50 is a semi-diagrammatic representation of a half- 

 larva at this stage, cut in the median sagittal plane. It 

 serves especially to illustrate the position and inter-relation- 

 ships of the ccelomic cavities at this stage. 



It is clear that very few changes are necessary in order to 

 change this larva into the free Actinotrocha with five pairs 

 of tentacles, the structure of which has been described in a 

 previous paper. In the epiblast the pre-oral sense-organ 

 and the subneural gland have yet to appear, and possibly a 

 proctodceum, whereas a pair of protocoelic pores do not seem 

 to have been yet formed. In the hypoblast the pleurochords 

 and the partial separation into pharynx and stomach are still 

 required. The later growth is largely a protuberance of the 

 anal papilla and the surrounding parts, and with it an 

 increase in size and extent of the metacoeles until they would 

 meet ventrally to form a ventral mesentery, whilst the peri- 

 anal band would form later. 



Metschnikoff (13) found pelagic Actinotrocha larvas 

 with no more than one pair of tentacles, and traced them up 

 to the later stages, so that the presence of a proctodeeura and 

 the fuller development of the collar nephridia are among the 

 few points still requiring elucidation. Metschnikoff's larvae 

 belong to one of the smaller species at Naples, and it is 

 important to note that they leave the tentacles of the parent 

 at a much earlier stage than is the case in Phoronis 

 Buskii or P. australis. 



