248 



CCELENTERATA. 



In many composite corals calcareous structures, as will be subse- 

 quently seen, are commonly developed externally to the thecae of 

 the polypes, constituting what is known by the general name of 

 "peritheca" or "exotheca." In a simple sclerodermic coral, such 

 as we are here considering, the only exothecal structures are the so- 

 called "costae." These are vertical ridges on the exterior of the 

 corallum, which correspond with the septa, and which, in fact, are 

 the exothecal edges of the septal laminae. In some cases, however, 

 we meet with similar vertical ridges (" rugae " or " pseudocostae ") on 

 the exterior of the theca which do not correspond with the septa 

 within, in many instances alternating with the latter ; but the true 

 nature of these is not clear. The costae, when present, vary much 

 as to their relative distance apart, their breadth, their solidity, and 

 their ornamentation with tubercles, granules, or teeth. 



In the interior of the corallum various " endothecal " structures 

 may be developed, all of which may be regarded as essentially pro- 

 duced by modifications of, or outgrowths from, the septa. In many 



Fig. 128. — A, Transverse section of a simple Zoantharian Coral [Caryophyllia Boiverbanki), 

 enlarged. (After Milne-Edwards and Haime.) /, Theca ; s, One of the primary septa _; p, One 

 of the "pali." In the centre is seen the irregular "columella." b, Transverse section of a 

 Rugose coral (Crepidophylhtm stcbccespitosztni), enlarged four times (Original). /, Theca ; d, 

 " Dissepiments" ; .y, One of the first order of septa ; s, One of the second order of septa; /, " Fos- 

 sula." In the centre of the coral is a space occupied by tabulae. 



corals the septal laminae themselves are more or less rudimentary, and 

 may be represented only by vertical rows of spines (as in Favosites) 

 or by mere longitudinal striae (as in species of Cystiphylluni). When 

 the septa have the form of complete calcareous lamellae, they may 

 terminate internally by free edges, and the centre of the visceral 

 chamber may be completely vacant. In other cases, narrow vertical 

 plates which are known as " pali " (fig. 128, a) are developed at the 

 inner ends of certain of the septa, and have the appearance of being 

 continuations of the latter, though detached from them. When a 

 " columella " is present, the inner edges of the pali are united with 



