CCELENTERATA I ACTINOZOA. 



of the ectoderm the " enderon" of Huxley and it is there- 

 fore termed " tissue-secretion " by Mr Dana. In the sclero- 

 dermic corallum each polype has a complete skeleton of its 

 own, and the entire coral may consist of one such skeleton, or 

 of several such united by the calcareous matter of the coenosarc. 



A sclerodermic corallum, therefore, like 

 the animal which produces it, may be simple 

 or composite, according as it is produced by 

 a single polype or by several united by a 

 coanosarc. It consists, therefore, of a single 

 calcareous cup, or " corallite \ " or of several 

 such united by a common calcareous bond 

 or basis, the "coenenchyma." Taking a single 

 " corallite " (fig. 42) as the type, we find that 

 it shows its origin and nature plainly in its 

 form. It consists of a cylindrical or conical 

 tube of carbonate of lime, the outer wall of 

 which is called the " theca." The upper 

 part of the space included by the " theca " 

 is vacant, and it is termed the cup or 

 " calice ; " but the lower part is subdivided 

 into a series of chambers, or " loculi/' by a 

 series of radiating, vertical, calcareous plates, 

 which are called the " septa " (figs. 42, 43). 

 The septa extend from the inner surface of the theca towards 

 its centre, where they usually unite to form an axial column, 

 called the "columella." Many of the septa, however, do 

 not reach the centre, but stop short at some distance from 





. Cyathaxonia 

 A simple 



sc'erodermic coral, 

 showing the theca, with 

 its cotae, the calice, 

 with the columella in 

 its centre, and the septa. 

 A portion of wall of the 

 theca is broken, in order 

 to show the interior of 

 the calice. 



Fig. 43. Diagrammatic sections of corals. A, Section of sclerodermic coral, showing 

 five primary septa, the columella, and costa; (c). B, Section of Rugosa coral, 

 showing four primary septa. Between the primary septa are seen the secondary and 

 tertiary septa. 



the columella, often being broken up into upright pillars, 

 called " pali." The parts thus described as essentially com- 

 posing a corallite in a typical sclerodermic corallum are 



