128 MANUAL OF ZOOLOGY. 



In Isis (fig. 33, b) and Mopsea the sclerobasis consists of alter- 

 nate calcareous and horny segments, branches being developed 

 in the former from the calcareous, and in the latter from the 

 homy segments. 



In Corallium rubrum, the "red coral" of commerce, Ihe 

 sclerobasis is unarticulate, or unjoin ted, and is entirely cal- 

 careous. It is the most familiar member of the family, and is 

 largely imported for ornamental purposes. Red coral consists 

 of a branched densely calcareous sclerobasis, which is finely 

 grooved upon its surface, and is of a bright-red colour. The 

 corallum is invested by a coenosarc, also of a red colour, 

 which is studded by the apertures for the polypes, which are 

 white, and possess eight pinnately- fringed tentacles. The 

 entire ccenosarc is channelled out by a number of anastomos- 

 ing 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 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 Jiugosa is highly developed, sclerodermic, 

 with true thecae, and oft«n presenting both septa and tabulae 

 combined. The septa are in multiples of four (fig. 32, b), 

 unlike the recent sclerodermic coralh, in which they are in 

 niultiples oi five or six. There is, further, no true ccenen- 

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

 posite, increasing either by parietal or by calicular gemmation. 

 Recently it has been shown that some very abnormal Rugose 

 corals were provided with a lid or operculum, closing the 

 mouth of the calice. In the genus Calceola, formerly referred 

 to the Brachiopoda, and very abundant in certain parts of the 



