308 MR. J. E. DrEEDEN OK 



The main point sought to be established is that the larva] spaces 

 of Lebrunia represent a paired coelome embryologically equivalent 

 to that of the higher Metazoa, and, consequently, that the 

 chambers of the adult are the same. For there is no doubt as to 

 the manner in whicb these latter arise from the primary spaces. 



It remains to discuss what support there is otherwise for such 

 a change in conception of the layers and internal cavity of the 

 Scyphozoa. 



Prof. J. P. McMurrich, in his most valuable series of '• Contri- 

 butions on the Morphology of the Actinozoa," devotes Part II. 

 (1891) to the development of the Hexactiniae, and therein 

 describes an almost complete series of stages from the egg to 

 the adult, taking his examples from the genera Metridium, 

 Sliodactis, and Aulactinia. In reviewing all the known cases of 

 early development, he concludes, as already mentioned, that the 

 so-called endoderm of the Actinozoa is derived from the hollow 

 blastula by the process of delamination, not by invagination, the 

 extent of its cellular development being mainly dependent upon 

 the amount of yolk present. 



EoUowing partly the results of MetschnikofF, McMurrich 

 regards the process of delamination as a modified form of the 

 more primitive process of immigration. In groups higher than 

 the Coelenterata the products of immigration, whether from 

 the ectoderm or endoderm or from both, are regarded as meso- 

 dermic {ex. the " mesenchyme " of Echinoderms). There is little 

 doubt that the internal parenchymatous tist^ue of the Lebrunia 

 larva has arisen by delamination in the same way as in other 

 Actinozoa : hence, if an independent endoderm, as from an 

 archenteric invagination, can be established, there seems no 

 reason why the first-mentioned tissue should not be regarded as 

 the homologue of the mesoderm of higher Metazoa. May we 

 not in Lebrunia have " mesenchyme " arising by delamination 

 (in migration), and " mesothelium " (endoderm) from an arch- 

 enteric invagination, as in the Echinoderms ? 



To the solid larval stage in the 7\ctinozoa, where segmentation 

 is completed and before the formation of the stomodseum 

 McMurrich (p. 310) applies the term " Sterrula." 



In many Alcyonaria, and apparently in some Madreporaria 

 {Manicina and BalanopTiyllia) , this is a solid mass of more or 

 let^s definite cells ; whereas in Metridium, so far as McMurrich's 



