THE EDWAUDSTA-STAGE OF LEBRUKIA. 3U3' 



mesenteries, give rise to another — the hexaineral. Even amongst 

 the Hexactiniae, however, octameral or tetrameral examples may 

 occur; occasionally in such a form as Aiptasia annulata. but 

 apparently always in most species of the genus Cbrynantis 

 (Duerden, 1898, p. 649). 



McMurrich (1891, p. 311) is inclined to regard the differences 

 of arrangement of the retractor muscles of the mesenteries, 

 such as one sees in the directives of the Alcyonaria and in the 

 Zoantharia, as of secondary importance, in comparison with the 

 order of development and number of mesenteries. 



Undoubtedly the character of greatest concern is the occurrence 

 in an Actinian of what appears to be a system of primitive 

 coelomic spaces connected with a closed archenteron, within an 

 otherwise solid interior. And it remains to be seen what support 

 there is for the views already advanced in regard to such. 



Until the earlier stages of the embryo have been obtained, it 

 is impossible to determine the manner in which the conditions 

 have arisen, and therefore to homologize them with certainty 

 with what occurs in other groups. It will be necessary to ascer- 

 tain whether an actual primary invagination takes place before 

 or after the appearance of the supporting lamella, and if the 

 spaces themselves are primitively formed as evaginations of this, 

 the limiting layer being a direct continuation of its walls ; or, 

 whether the spaces originate independently as splittings within 

 the solid tissue, their limiting layer then arising as a modifica- 

 tion of the marginal cells, and later entering into communication 

 with the internal end of the archenteron. 



The evidence obtainable from the development of other Scy- 

 phozoa assists but little in the elucidation of the actual facts 

 presented hj Lehrunia at the stages under consideration. From 

 the fertilized ovnm a blastosphere results, and, according to the 

 accounts of some observers, the two-layered planulais formed by 

 invagination, and, according to others, by delamination. Heview- 

 ing all the known cases, McMurrich (L891), in the light of his 

 own results with Metridium, comes to the conclusion that, with the 

 probable exceptions of Pelagia and Nausithoe, in every case the 

 endoderm in the Scyphozoa is produced by delamination ; that 

 the results of Kowalewski with Actinia (sp. ?) and CeriantJius, 

 of Jourdan with Actinia equina, as also of Haeckel with the 

 Alcyonarian Monoxenia. in that they ascribe the production of 



23* 



