CCELENTERATA : ACTINOZOA. 



121 



responds to " that part of the enderon wjiich forms the floor of 

 the somatic cavity below the digestive sacj" whilst the "septa" 

 correspond to the " mesenteries, " and, hke them, are called 

 " primary " and " secondary," according as they reach the colu- 

 mella or fall short of it. When there are several corallites, the 

 bond of union between them, the "ccenenchyma," is secreted by 

 the " ccenosarc," to which it corresponds. In many Actinozoa, 

 however, the sclerodermic corallum is not present in the typical 

 form above described, but simply in the form of calcareous 

 spicules or nodules scattered through the tissues of the animal. 

 There are, also, members of the class in which both a scleroder- 

 mic and a sclerobasic corallum are present, the latter constituting 

 the main skeleton, whilst the former is represented by scattered 

 spicules. The coral tissue itself is known as " sclerenchyma," 

 and it varies considerably in texture, being sometimes extreme- 

 ly compact, and at other times very loosely put together. 



From what has been said it will be seen that a sclerobasic 

 corallum can easily be distinguished from a sclerodermic by 

 inspection ; the former (fig. 33, V) being usually more or less 



Ffe- 33' — Sclerodermic and Sclerobasic Corals, a Portion of brancli qI DendropkylUa 

 nigrescens, a sclerodermic coral (after Dana); b Longitudinal section of IsU 

 kippuris, a sclerobasic coral, exhibiting the external bark or cosnosarc, with its em- 

 bedded polypes, supported by the internal axis or skeleton (after Jones). 



smooth, and being invariably devoid of the cups or receptacles 

 for the separate polypes, which are always present in the latter 

 (fig- 33> '^- The more important variations of detail which 

 occur in both classes of corals will be noticed under the differ- 



