CCELENTERATA : RUGOSA. 173 



CHAPTER XIV. 



RUGOSA. 



THE members of this order agree with the Zoantharia sclero- 

 dermata in possessing a well-developed sclerodermic corallum, 

 with a true theca, but generally possessing both tabulcz and septa 

 combined. The septa, however, are generally (though apparently 

 not always} some multiple of four, and there is commonly a single 

 predominant septum (sometimes three such}, or a vacant space 

 (fossula) representing such a septum. Some of the Rugosa are 

 simple, others are compound ; but the latter are destitute of a 

 true coenenchyma. The mode of increase in the compound 

 forms is principally by calicular gemmation, or by lateral 

 budding. 



There are only two living genera of corals (viz., the Guynia 

 of the Mediterranean and the Haplophyllia of Florida) which 

 agree with the Rugosa in the tetrameral arrangement of the 

 septa ; and it is doubtful whether we are justified in asserting 

 positively on this ground alone that these genera really are 

 Rugose corals. We have, therefore, simply to consider very 

 briefly the corallum of the Rugose corals, which alone has 

 been preserved to us in a fossil condition. In its most essen- 

 tial respects, the corallum of the Rugosa is quite identical with 

 that of the typical Zoantharia sclerodermata. In both alike 

 the corallum may be simple or compound ; in both alike the 

 simple form of corallum (fig. 86) consists of an outer wall or 

 "theca," enclosing a central space or "visceral chamber," 

 which is divided into compartments by a series of radiating 

 lamellae or "septa;" in both alike the structures known as 

 " dissepiments," " tabulae," and " columella," may be de- 

 veloped ; and in both alike the compound corallum may be 

 regarded as a variously-formed aggregate of " corallites," simi- 

 lar in their fundamental structure to the simple corallum. 



On the other hand, the corallum of the Rugosa exhibits the 

 following more striking points of difference as compared with 

 that of the Zoantharia sclerodermata: (i.) The septa appear 

 to be primitively developed in four systems, instead of six or 

 five. Sometimes the adult corallum (as in Stauria} exhibits 

 the four primitive septa in a pre-eminently developed condi- 

 tion, but this is not commonly the case. (2.) The septa are 

 rendered more or less irregular in their arrangement by the 

 presence of a curious vacant space (sometimes three or four), 



