MADREPORARIA RUGOSA. 



277 



ous and so important, that it is impossible to imagine that the coralla 

 in the two cases were secreted by different methods, or bore dis- 

 similar relations to the soft parts of the animals producing them. 

 Thus, in both groups alike the simple form of corallum (fig. 155) 

 consists of an outer wall or "theca," enclosing a central space or 

 "visceral chamber," which is ordinarily divided into a series of 

 compartments by vertical partitions or "septa"; in both alike the 

 " visceral chamber" may be partitioned off into storeys by horizontal 

 plates or " tabulae " ; in both alike the interseptal loculi are liable to 

 be more or less subdivided by " dissepiments " ; and in both alike 



Fig. 155. — Morphology of the Rngosa. A, Fragment of Zaphrentis gigantea, showing the 

 septa (s), with the sparse dissepiments crossing the interseptal loculi, the epitheca (e), and the 

 thin proper wall (w) ; B, Transverse section of Zaphrentis Gt(erangeri, showing the septa and 

 dissepiments, the central area occupied solely by the tabulae, and the "fossula" (/); c, Longi- 

 tudinal section of the last, showing the arrangement of the tabulae, (a is after Edwards and 

 Haime ; b and c are after Mr James Thomson.) 



the axial rod, known as the "columella," may be developed. In 

 both groups, moreover, the corallum is often composite, and may 

 be regarded as a variously formed aggregate of " corallites," each 

 of these subordinate elements of the colony being essentially similar 

 in structure to the typical simple corallum. 



In spite of the above-mentioned general resemblances, it will 

 be found, however, that the central and most typical group of the 

 old order Rugosa — viz., the group of the Zaphrentoid corals — is 

 separated from the division of the Madreporaria Aporosa by im- 

 portant morphological and developmental characters. But the 

 Zaphrentoids are very intimately connected by many intermediate 

 links with the Cyathophylloidea, the group of Rugosa which most 

 nearly approaches the Astrceidce. The Cyathophylloids, again, have a 



