süBCLÄSS II HEXACORALLA 89 



Bryozoa, and Echinoderms. The Tetracoralla are not less conspicuous in the 

 Devonian, especially in the Middle and Upper Devonian of the Eifel district, 

 Westphalia, Nassau, Harz, Boulogne, England, and North America. ■- Particu- 

 larly abundant here are the genera Cyathophyllum, Campophyllum, Zaphrentis, 

 Cystiphyllum, Phülipsasti'ea, Calceola, etc. Zaphrentis, Amplexus, Lithostrotion, 

 Lonsdaleia, Cyclophyllum^ etc., predominate in the Carboniferous Limestone of 

 Belgium, England, Ireland, and North America ; while in the Zechstein the 

 solitary genus known is Polycoelia. On the other band, the Permo-Carboni- 

 ferous rocks of the Salt Range in India and of the island of Timor contain the 

 genera Zaphrentis, Amplexus, Clisiophyllum, and Lonsdaleia. According to Frech, 

 the genera Gigantostylis, Piriacophyllum, and Coccophyllum, occurring in the Alpine 

 Trias, belong to the Tetracoralla ; and to this group also have been assigned 

 Holocystis E. and H., from the Cretaceous, and the recent genera Haplophyllum 

 Pourtales, and Guynia Duncan. A number of Paleozoic Tetracoralla, such as 

 Baitersbyia, Heterophyllia, and Stauria, are referred by Duncan and Nicholson to 

 the Hexacoralla (^' Astraeidae"). 



Subclass 2. HEXACORALLA Haeckel. 



(Zoantharia Blainville ; Hexadinia and Polyadinia Ehrenberg.) 



Simple or composite polyps, with radial mesenferies arising in cydes of six, iwelve, 

 or multiples of six (more rarely pentameral, septameral or odameral) ; frequently with 

 calcareous corallum, hut sometimes fleshy or with hoo'ny axis. 



To the Hexacoralla belong the calcareous reef-building and deep-sea corals 

 (Madreporaria) of the present day, the fleshy sea-anemones (Adiniaria), and 

 those forms characterised by the secretion of a horny axis (Antipatharia). Of 

 these three Orders, only the Madreporaria are known in a fossil State. These 

 forms are distinguished from the Tetracoralla by the hexameral System and 

 radial arrangement of mesenteries and septa ; and from the Octocoralla, in 

 addition to the above-named characters, by their simple tentacles. 



According to Duerden, either before or shortly after extrusion of the 

 larva, the six primary pairs of mesenteries (protocnemes), constituting the first 

 cycle, make their appearance. The organs arise in bilateral pairs, in a 

 regulär and well-defined order, which is uniform for all the species yet 

 studied. The first two or three pairs arise around the oral extremity of the 

 larva, while the others first appear at varying distances down the wall. The 

 protocnemic sequence is represented by the Roman numerals in Fig. 126, and 

 agrees with that established for the greater number of actinians. The first 

 four pairs very early unite with the stomodaeum, but the fifth and sixth 

 pairs remain free or incomplete for a lengthened period, suggesting a diflferent 

 phylogenetic significance from the others. 



The six pairs of second cycle mesenteries (metacnemes) arise after fixation, 

 but in a manner altogether difFerent from that followed by the first cycle. 

 They appear on the polypal wall in unilateral pairs or couples within the 

 six primary exocoeles, and in a succession which is from the dorsal to the 

 ventral side of the polyp, not the whole cycle at a time. For a long time, 

 as shown in Fig. 127, the six pairs present a diff"erence in size, corresponding 

 with their dorso-ventral or antero-posterior order of appearance. 



