THE EDWAEDSIA-STAGE OF LEBETJNIA. 277 



and late larvae, I refer to phases reached in the development, 

 independently of whether the examples were or were not ex- 

 truded. Practically all the material was utilized for purposes 

 of study. 



Succinctly the general facts of the anatomy and histology are 

 as follows :— 



The ectoderm is a thick layer with the constituent elements — 

 ciliated supporting cells, nematocysts in all stages of develop- 

 ment, gland-cells, and nerve-elements — well differentiated. The 

 supporting lamella contains isolated cells, and a definite, though 

 very weak, musculature is already formed on its inner surface, 

 its fibres arranged in a circular manner. 



Eight mesenteries are present and extend the whole length of 

 the larva ; in the upper stomodaeal region they are all perfect 

 and arranged tetramerally *. 



The stomodseum at first is a very short, strongly ciliated, 

 rounded tube, without any external aperture. Internally it 

 communicates with four radiating canals, which are connected 

 with two sinuses, and these terminate in a system of inter- 



* Such a stage in which only eight perfect mesenteries are present, and in 

 which no further numerical increase takes place for some time, has been found 

 to be very constant in the development of the Zoantharia. The mesenteries 

 are those of the first four pairs, and consist of two lateral pairs and two 

 median pairs — the directives. The retractor muscles on the two lateral pairs 

 occur on the face of the mesenteries turned towards one pair of the directives, 

 which, following the terminology of Haddon, is the sulcar or ventral. The 

 other pair of directives is the sulcular or dorsal, and in these the retractor 

 muscles are likewise on the face directed towards the sulcar aspect of the 

 polyp. 



The pair of lateral mesenteries next the sulcar end — sulco-lateral — are the 

 first to appear in the embryo ; then, in the majority of cases, the lateral pair 

 next the sulcular end — sulculo-lateral ; the sulcar directives constitute the third 

 pair, and the sulcular the fourth in the order of development. 



Lacaze-Duthiers first recognized the octoradiate condition as one of the 

 resting stages in the development of the Actinias. Haddon (1889) described it 

 as the third stage of mesenterial development, at the same time realizing its 

 phylogenetic importance and similarity to the permanent condition in the 

 EdwardsicB ; McMurrich (1889) obtained it 'in Aulactinia stelloicles, and later 

 in others, and applied the term " Edwardsia-stage," an apt designation since 

 adopted by all writers on the subject. The researches of these authors, and of 

 H. V. Wilson, Boveri, van Beneden, G. von Koch, and others, demonstrate that 

 the phase is repeated in the ontogeny of such diverse groups as the Ceriantheffi, 

 Zoanthese, Hexactinise, and the Madreporaria. 



