CCELENTERATA : ACTINOZOA. 1 29 



Devonian System, the operculum consisted of a single valve or 

 piece. In Goniophyllum four valves were present, and in 

 Cystiphyllum prisiiMticum there were four or more valves in 

 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 

 change, however, cannot be accepted without the production 

 of very conclusive evidence in its favour. A strong argu- 

 ment against referring the Rugose and Tabulate Corals, as 

 proposed by Agassiz, to the Hydrozoa, is their possession in 

 most cases of well-developed septa, impl3ang, of course, the 

 existence in the living animal of mesenteries, structures which 

 are wholly wanting in the Hydrozoa. 



Distinctions between the Soralla of the Orders of 

 Actinozoa. — Having now conadered all the orders of the 

 Actinozoa in which coralla ar J| developed, it may be as well 

 briefly to review their m( ye« g ggy ng 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 Antipathidm 

 z.vAHyalonem.adce{T) amongst the Zoantharia, and in the families 

 Pennatulidm and Gorgonidce amongst the Akyonaria ; the fol- 

 lowing being the differences between them : — 



1. Antipathidce. — Sclerobasis spinulous or smooth ; tentacles 

 and soft parts in multiples of six. 



2. HyalonemadcB (?). — Sclerobasis siliceous, composed of 

 numerous threads ; tentacles in multiples of five. 



3. PennatulidcB. — Sclerobasis sulcate, free ; soft parts in mul- 

 tiples of four. 



4. GorgonidcB. — Sclerobasis sulcate, attached proximally ; 

 soft parts in multiples of four. 



Sclerodermic coralla fall under two heads, according as they 

 are simply composed of scattered spicules, or are provided with 

 'true theccB. 



I. Spicular coralla occur in the Zoantharia Malacodermata 

 (occasionally), and in the Alcyonida; and no differences can 

 be stated between the coralla themselves. The animals, how- 

 ever, differ entirely, the soft parts of the former being in mul- 



