CCELENTEKATA : ACTINOZOA. 109 



canals, which communicate with the somatic cavities of the 

 polypes, and are said to be in direct communication with the 

 external medium by means of numerous perforations in their 

 walls. The entire canal system is filled with a nutrient fluid, 

 containing corpuscles, and known as the ' milk.' 



CHAPTER XV. 

 RUGOSA. 



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

 extinct, and, with the single exception of Holocystis elegans 

 from the Lower Cretaceous Rocks, 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 theeoe, 

 and often presenting both septa and tabulae combined. The 

 septa are in multiples of four (Jig. 27 fc), unlike the recent 

 sclerodermic coralla, in which they are in multiples, of Jive or 

 six. There is, further, no true ccenenchyma. Some of the 

 Rugosa are simple ; but others are composite, 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 pro- 

 duction of very conclusive evidence in its favour. 



DISTINCTIONS BETWEEN THE CORALLA OP THE ORDERS OF Ac- 

 TINOZOA. Having now considered all the orders of the Ae- 

 tinozoa 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 dis- 

 tinguished by inspection from a sclerodermic corallum by the 

 fact that the latter, unless composed simply of spicules, pre- 

 sents the cups or ' thecse,' in which the polypes were con- 

 tained ; the surface of the former being invariably destitute of 

 these receptacles. 



A sclerobasic corallum is found in the families Antipatliidm 

 and Hyalonemadoe amongst the Zoantharia, and in the families 

 PennatulidcB and Gorgonidce amongst the Alcyonaria ; the fol- 

 lowing being the differences between them : 



