180 FOSSIL ACTINOZOA. 



together by a greater or less quantity of calcareous matter 

 secreted by the ccenosarc. The entire compound corallum 

 consists, therefore, of a greater or less number of " corallites " 

 bound together by a calcareous basis, which is secreted by 

 the ccenosarc, and is called the " coenenchyma." In practice, 

 however, this theoretical view of the subject is not always 



Fig. 69. Caryophyllia borealis. A simple sclerodermic coral, twice the natural size. 

 Recent. (After Sir Wyville Thomson.) 



borne out. The compound corallum may, and often does, 

 consist of a number of corallites produced by budding or 

 cleavage from a primitive corallite, having their outer walls 

 amalgamated, or more or less completely free, but not sunk in 

 any general ccenenchyma. In other cases, the ccenenchyma, 

 though not actually absent, is very much reduced in quantity. 

 To comprehend the more intimate structure of a sclero- 

 dermic coral, we may take a simple corallum, such as figured 

 in fig. 69. Typically, such a coral is conical in shape, some- 

 times discoid, sometimes cylindrical, but in all cases possess- 

 ing an external " wall " or " theca," with an internal included 

 space. The theca may be very imperfect, often porous or 

 cribriform (" Perforate Corals "), or it may be strengthened by 

 a secondary calcareous investment, termed the " epitheca." 

 The " theca " encloses p, larger or smaller space which is 



