CCELENTERATA I ACTINOZOA. 



141 



the operculum. It is worthy of notice that some recent corals 

 (species of Primnoa, Paramuricea, and others) exhibit also a 

 more or less complete operculum. According to Professor 

 Agassiz, the Rugosa and the Tabulate division of the Zoantharia 

 ought not to be considered as belonging to the Actinozoa, 

 but should be placed amongst the Hydrozoa. This radical 



Fig. 50. Strombodes pentagonus. A 

 Silurian Rugose Coral. 



Fig. 51. Calceolasandalina. 

 An operculate Rugose Coral. 

 Devonian. 



change, however, cannot be accepted without the production 

 of very conclusive evidence in its favour. A strong argument 

 against referring the Rugose and Tabulate Corals, as proposed 

 by Agassiz, to the Hydrozoa, is their possession in most cases 

 of well-developed septa, implying, of course, the existence in 

 the living animal of mesenteries, structures which are wholly 

 wanting in the Hydrozoa. 



DISTINCTIONS BETWEEN THE CORALLA OF THE ORDERS OF 

 ACTINOZOA. Having now considered all the orders of the 

 Actinozoa in which coralla are developed, it may be as well 

 briefly to review their more striking differences. 



In the first place, a sclerobasic corallum may be distin- 

 guished by inspection from a sclerodermic corallum by the 

 fact that the latter, unless composed simply of spicules, 

 presents the cups or "'thecse/' in which the polypes were 

 contained ; the surface of the former being invariably destitute 

 of these receptacles. 



A sclerobasic corallum is found in the families Antipathidce 

 and Hyalonemadce (?) amongst the Zoantharia, and in the 

 families Pennatulidcz and Gorgonidce amongst the Alcyonaria ; 

 the following being the differences between them : 



1. Antipathid(E. Sclerobasis spinulous or smooth; tentacles 

 and soft parts in multiples of six. 



2. Hyalonemadce (.?). Sclerobasis siliceous, composed of 

 numerous threads ; tentacles in multiples of five. 



