THE EDWARDSIA-STAGE OF LEBEUjSIA. 309 



embryos allowed the study to be continued, the sterrula was 

 provided with only a comparatively narrow layer o£ endoderm, 

 the remainder of the cavity being iilled with yolk-granules. In 

 the solid sterrula of the Alcyonaria the middle cellular tissue 

 early begins to disorganize as the growth of the larva proceed?, 

 giving rise, of course, to the usual coelenterate gastro-coelomic 

 cavity with a unilaminar epithelium; while in the other cases it 

 seems that the coelenteron is produced by the absorption of the 

 jo\k, or some of the latter may be extruded through the mouth 

 of the embryo. 



The earliest larva of Lebrunia I possess has passed beyond the 

 sterrula stage, the mesenteries and the archenteron being already 

 formed. It is in the late stage to which the solid cellular tissue 

 persists, that the species appears to differ from other examples 

 yet recorded. And it would appear to be this entire retention 

 of the central tissue, as compared with its early disappearance 

 or absence in other Scyphozoa, which may enable the morpho- 

 logical conceptions of the other systems to be obtained in such a 

 way as is known for no other species. 



Is there any evidence that such a system of diverticula occurs 

 at any stage in other Actinozoa ? For it can scarcely be supposed 

 that such an apparently fundamental phenomenon is restricted to 

 an isolated type. 



Pew Zoantharia larvae of exactly the same stage as the earliest 

 Lebrunia have been minutely described. In some respects, as in 

 the stage reached in the development of the mesenteries, the 

 Lebrunia larva is far advanced ; but in others — the persistence 

 of the vacuolated tissue and non-formation of oral aperture — it 

 is somewhat early in its development. McMurrich (1891) 

 found in the youngest embryos oi Rhodactis Sancti-Tliomee that 

 the so-called endoderm-celLs completely fill the central cavity, 

 and show little or no arrangement into a definite layer. At the 

 stage, however, where only two mesenteries are present, a well- 

 marked central cavity already existed below the upper region of 

 the body, though the endoderm above was yet solid, no inter- 

 niesenterial cavities having appeared. In this species then the 

 solid endoderm begins to disorganize at a much earlier stage 

 than in Lebrunia, in fact before the mesenteries, witb which the 

 spaces are associated, are formed. McMurrich's earliest stages 

 of Aulactinia possessed eight perfect mesenteries, the first only 



