278 ME. J. E. DUEEDEN ON 



mesenterial and mesenterial spaces. The spaces througliout 

 have a distinct ciliated limiting layer, in continuity with the 

 lining of the stomodseum. Otherwise the whole o£ the interior 

 is filled with a vacuolated, undifferentiated tissue, provided with 

 nuclei and zooxanthellse, but devoid of yolk. Such a larva may 

 therefore be regarded as a nearly solid mass of cells. 



Between the earliest larvae and the latest, changes o£ great 

 significance have been effected, in addition to those involved in 

 the production of the tentacles. 



The middle vacuolated tissue has broken down and a gastro- 

 ccelomic cavity of the usual Coelenterate type is in process of 

 formation ; into this the stomodseum, now provided with an oral 

 aperture, depends and opens directly. The primary spaces are 

 in process of giving rise to the mesenterial chambers of the 

 adult. 



One pair of the original eight mesenteries — the sulco-lateral — 

 remains connected with the stomodseum longer than the rest, and 

 each of the pair bears mesenterial filaments in direct continuity 

 with the stomodseal lining. Only the median streak is as yet 

 developed. 



Of the many expelled larvae secured, only two or three retain 

 the solid phase in its perfect condition, with the stomodseum and 

 its diverticula intact. The remainder, including the specimens 

 preserved within the body of the adult, have passed into the 

 second stage in which the middle tissue is becoming disorganized. 



The dififerent structures will now be described in greater 

 detail. 



1. Ectoderm. 



The ectoderm is an exceptionally broad layer, uniformly ciliated 

 throughout, with its free margin very pronounced in sections 

 stained in borax-carmine or hsematoxylin (PL 18. fig. 8). This latter 

 character is due to the occurrence of a very delicate rectangular 

 enlargement at the base of each cilium ; the closeness of these and 

 the readiness with which they take up colouring-matter produce 

 a strong delimitation from the rest of the layer. The Hertwigs 

 (1879, Taf. iii. figs. 5-8) figure a similar appearance at the edge 

 of the ciliated ectoderm of the external surface and stomodseum 

 of Antliea cereus. 



Most of the nuclei of the various cellular constituents are 

 arranged in a broad zone a little within the periphery, but a few 



