CCELENTERATA : ACTINOZOA. 1 15 



simple or composite, according as it is produced by a single 

 polype or by several united by a coenosarc. It consists, there- 

 lore, of a single calcareous cup, or "corallite;" or of several 

 such united by a common calcareous bond or basis, the 

 "ccenenchyma." Taking a single "corallite" (fig. 30, d) 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 sub- 

 divided into a series of chambers, or " loculi," by a series of 

 radiating, vertical, calcareous plates, which are called the 

 " septa " (fig. 30, b). 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 the columella, often being broken up into 

 upright pillars, called "pali." The parts thus described as 

 essentially composing a corallite in a typical sclerodermic 

 corallum are related in the most obvious manner to the soft 

 structures of the animal by which they are secreted. Thus, 

 the " theca" clearly corresponds to the " column-wall," or the 

 general wall of the body; the " columella," when present, cor- 

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

 the somatic cavity below the digestive sac," whilst the "septa" 

 correspond to the "mesenteries," and, like 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 " coenenchyma," is secreted 

 by the " coenosarc," to which it corresponds. In many Actin- 

 ozoa, however, the sclerodermic corallum is not present in the 

 typical form above described, but simply in the form of cal- 

 careous spicules or nodules scattered through the tissues of 

 the animal. There are, also, members of the class in which 

 both a sclerodermic 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 tex- 

 ture, being sometimes extremely 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. 31, b} being usually more or less 

 smooth, and being invariably devoid of the cups or receptacles 

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



