THE ANTHOZOA 49 



Omdis diomedeae has been described by M'Murrich (76). It 

 has a sulcus but no sulculus ; eight complete mesenteries are 

 present, having an Edwardsia arrangement, and all bear gonads 

 and mesenterial filaments. In addition there are twelve micro- 

 mesenteries, whose arrangement recalls that observed in Gonactinia, 

 but there is an additional couple in each of the transverse chambers 

 (see Fig. XXII. 6). 



From the persistence of the Edwardsian mesenteries as 

 macromesenteries in Gonactinia and Oractis, and from the easy 

 transition from Gonactinia to Scytophorus, it may be concluded 

 that these forms have descended from an Edwardsia-like ancestor, 

 diverging somewhat low down from the line of descent which led 

 to the Hexactinian type. 



All the Zoantharia hitherto considered agree in their funda- 

 mental histological characters, and in spite of the exceptions 

 enumerated, it may be stated of them that, after the first cycle of 

 twelve couples of mesenteries is established, new mesenterial couples 

 are formed in the exocoeles between the couples already existing. 



The remaining groups of the Zoantharia offer greater diffi- 

 culties. They differ from the Actinian type both in histological 

 characters and in the disposition and sequence of their mesen- 

 teries. There are three groups to be considered — the Zoanthidea, 

 the Cerianthidea, and the Antipathidea. 



The Zoanthidea are mostly colonial, more rarely solitary 

 Zoantharia, without a skeleton, but encrusted externally by a coat 

 of sandy and other adventitious particles. The colonial forms 

 are united by basal stolons, which, like those of the Alcyonaria, 

 contain numerous solenia. The stolons may fuse to form a 

 membranous expansion, which again may be thickened' to form a 

 coenenchyme, in which the proximal moieties of the zooids are 

 imbedded. The external characters of the zooids are Actinia- 

 like. There are two circles of tentacles — an inner larger and 

 a smaller outer circle. The large and small tentacles alter- 

 nate with one another, and those of the one cycle are placed over 

 the exocoeles, those of the other over the entocoeles, so that there 

 are as many tentacles as mesenteries. The mesogloea differs 

 from that of all other Zoantharia in being permeated by canals 

 Avhich are filled with cells of ectodermic origin. There is a sulcus, 

 but no sulculus. The mesenteries of the Zoanthidea are bilaterally 

 disposed in a characteristic manner. 



There are two kinds of mesenteries — complete macromesen- 

 teries bearing gonads and filaments, and incomplete micromesen- 

 teries devoid of gonads and filaments. There are two couples of 

 directives with muscle banners turned away from one another. 

 The sulcar directives are macromesenteries, the asulcar directives 

 micromesenteries. Between these, on either side, lie a variable 



