MADREPORARIA APOROSA. 263 



lateral mesenteries) which develop reproductive organs. These two 

 mesenteries are much longer than the others, and correspond to inter- 

 mesenteric pouches of excessive depth, which fit into deep conical pits in 

 the skeleton, bounded by septa of much reduced size. In accordance 

 with these observations, it may be assumed that the "fossula" of the 

 Rugose Corals served to lodge a special hypertrophied mesentery, or 

 group of mesenteries, carrying reproductive organs. It is noteworthy, 

 however, that in the recent Seriatopora (as also in Madrepora Durvillei) 

 the two mesenteries which are modified for reproductive purposes are on 

 opposite sides of the polype j whereas the fossula of the Rugosa is usually 

 unilateral. 



As regards the classification of the Zoantharia, the order may be 

 divided into the three groups or sub-orders of the Acti?iiaria, Anti- 

 patharia, and Madreporaria. The first of these comprises the Sea- 

 anemones ; and, from the absence of a corallum in these forms, is 

 sometimes spoken of under the name of Zoantharia malacodermata. 

 The second group — sometimes called Zoantharia sclerobasica — com- 

 prises the Antipathidce or " Black Corals " and their allies, in 

 which a horny sclerobase is developed. Lastly, the great series 

 of the Madreporarians — sometimes spoken of as the Zoantha7-ia 

 sclerodermata — comprises all the ordinary " Corals " of the present 

 day along with the extinct group of the Rugosa, in all of which a 

 well-developed sclerodermic corallum is present. 



All the Zoantharians are marine, and from their common posses- 

 sion of a calcareous skeleton they have been largely preserved in the 

 fossil condition. Owing to their want of hard parts, the Sea-anem- 

 ones (Actim'aria) have left no traces of their past existence, and 

 need, therefore, no further consideration here. The group of the 

 A?itipatharia also requires merely to be mentioned, as no fossil 

 forms have been hitherto identified, the Leiopathes vetusta of Milne- 

 Edwards and Haime appearing to be a Gorgonian and therefore 

 referable to the Alcyonaria. There remains the great group of the 

 Madreporaria, which is represented in past time by a vast series 

 of fossil forms, and which requires, therefore, a somewhat detailed 

 examination. 



Madreporaria. 



The group of the Madreporarians or Zoantharia sclerodermata 

 comprises those Zoantharians in which a well-developed sclerodermic 

 corallum, which is not simply spicular, is present. The septa of the 

 corallum are, typically, arranged according to either a hexameral or a 

 tetrameral system ; but in some cases some other numerical law may 

 govern the disposition of the septa, and in other cases no special 

 symmetry may be recognisable. The organism may be simple, con- 

 sisting of a single polype only, or composite, consisting of many 



