26 1 



CHAPTER XVII. 



CHARACTERS AND DIVISIONS OF THE 

 ZOANTHARIA. 



MADREPORARIA APOROSA. 



From a palaeontological point of view the order Zoa?itharia is not 

 in a satisfactory position, since the only general definition of it 

 which can be given is one based upon the soft parts, and therefore 

 depending mainly upon characters which cannot be recognised in 

 the fossil forms. The living Zoantharians possess simple, usually 

 numerous te?itades, and the mesenteries are never eight in number. 

 The mesenteries typically exhibit a more or less clearly recognisable 

 bilateral arrangement, and usually show a hexameral symmetry ; 

 but some forms are completely radial, and the disposition of the 

 mesenteries in hexameral cycles is often departed from. In the 

 section of the Rugose Corals, more particularly, the symmetry is 

 typically tetrameral. A corallum may be wholly wanting (Actini- 

 aria), or a horny sclerobasis may be developed (Antipatharid). In 

 most Zoantharians (Madreporarid) there is, however, a well- devel- 

 oped sclerodermic corallum, the symmetry of which varies in accord- 

 ance with that of the mesenteries. 



The general arrangement and structure of the soft and hard 

 parts of the Zoantharians have been sufficiently discussed in dealing 

 with the Actinozoa as a whole ; but there are some points connected 

 with the symmetry and arrangement of the mesenteries which may 

 be briefly alluded to here : — 



Hexameral symmetry obtains as a general rule, though not uni- 

 versally, in the Acti?iid<z (Sea-anemones), Antipathidce, Madreporaria 

 Perforata, M. Eungida, and M. Aporosa ; the mesenteries and 

 septa being typically arranged in cycles of six pairs each (fig. 138). 

 On the other hand, tetrameral symmetry is commonly recognisable 

 in the Rugose Corals, though in some of the forms generally in- 



