No. 3-] MORPHOLOGY OF THE ACTINOZOA. 32 1 



edge of the stomatodasum, similar to what I have just described, 

 occur, and apparently have had something to do with the forma- 

 tion of the " Flimmerstreifen " of the filaments. These struc- 

 tures, however, have not made their appearance in any of the 

 RJiodactis embryos I have had for study. 



In another specimen, in which the second pair of mesenteries 

 was just forming, I found some distance up upon the endoder- 

 mal surface of the stomatodxum a patch of ectoderm (Fig. 18). 

 It was not situated symmetrically, but was close to one of 

 the mesenteries of the first pair. I thought at first that this 

 might be a piece of reflected ectoderm which had migrated up- 

 wards some distance. It had evidently, however, completely 

 lost its continuity with the stomatodaeal ectoderm below, while 

 on the other hand the mesogloea, which ought to have occurred 

 below it, separating it from the stomatodaeal ectoderm, was not 

 present, so that it was in direct continuity with the stomato- 

 daeal ectoderm, and it seems as if there had been an escape of 

 ectoderm upon the endodermal surface of the stomatodaeum 

 through a perforation of the mesogloea. I am strongly inclined 

 to take this to be a malformation, especially as we have already 

 seen that the filaments of the second pair of mesenteries develop 

 only after the mesenteries have become perfect, there being 

 therefore no necessity for an ectodermal reflection, even though 

 they are developed from that layer. 



The evidence, so far as it goes, indicates that in RJiodactis 

 there is no reflection of the stomatodaeal ectoderm in connec- 

 tion with the formation of the mesenterial filaments, and in 

 Aulactinia the evidence points to the same direction. The 

 question arises as to whether the arrangement in Manicina is 

 the more primitive, or whether the formation of the filaments 

 is typically postponed until the mesenteries have become per- 

 fect. The latter seems to me to be the more primitive method, 

 since in all probability the mesenteries are phylogenetically 

 older structures than the filaments. In embryos of Edivardsia, 

 too, I find that the filaments, with the exception perhaps of 

 those of the first pair of mesenteries, do not appear till the eight 

 mesenteries have become perfect. This has an important bear- 

 ing upon the question, since Edivardsia must be regarded as 

 representing an ancestral condition of the Hexactinians. I 

 think, then, that the peculiarities of Manicina are secondary, 



