CCELENTERATA : ACTINOZOA. 123 



CHAPTER XV. 

 RUG OS A. 



ORDER III. RUGOSA. The members of this order are entirely 

 extinct, and, with the exception of Holocystis elegans from the 

 Lower Cretaceous Rocks, and a few more modern forms, are 

 not known to occur in deposits younger than the Palaeozoic 

 epoch. With the soft parts of the Rugosa we are, of course, 

 entirely unacquainted, and the definition of the order must, 

 therefore, be founded upon the characters of the corallum. 

 The corallum in the Rugosa is highly developed, sclerodermic, 

 with true thecae, and often presenting both septa and tabulae 

 combined. The septa are in multiples of four (fig. 30, b\ 

 unlike the recent sclerodermic coralla, in which they are in 

 multiples of five or six. There is, further, no true ccenen- 

 chyma. Some of the Rugcsa are simple ; but others are com- 

 posite, increasing either by parietal or by calicular gemmation. 

 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. 



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 " thecae," 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 Antipathida 

 and Hyalonemadce amongst the Zoantharia, and in the families 

 Pennatulidce and Gorgonidcz amongst the Alcyonaria; the fol- 

 lowing being the differences between them : 



1. Antipathidce. Sclerobasis spinulous or smooth; tentacles 

 and soft parts in multiples of six. 



2. Hyalonemada. Sclerobasis siliceous, composed of nume- 

 rous threads ; tentacles in multiples of five. 



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

 tiples of four. 



