THE CORALLIGENA. 



139 



tinozo(5n, are often different from the rest. Each mesentery 

 ends, at its aboral extremity, in a free edge, often provided 



Fig. 29.~Perpendicular section of Actinia holsatica (after Prey and Leuckart).— a, 

 mouth ; 6, gastric cavity ; c, common cavity, into wiiicb the gastric cavity and 

 the intermesenteric chambers open ; d, iutermesenteric chambers ; e, thickened 

 free margin, containing thread-cells of, /, a mesentery; g, reproductive organ ; A, 

 tentacle. 



with a thickened and folded margin ; and these free edges 

 look toward the centre of an axial cavity,^ into which the gas- 

 tric sac and all the intermesenteric chambers open. 



In the CoralUgena, the outer wall of the body is not pro- 

 vided with bands of large paddle-like cilia. Most of them are 

 fixed temporarily or permanently, and many give rise by 

 gemmation to turf-like, or arborescent, zoanthodemes. The 

 great majority possess a hard skeleton, composed principally 

 of carbonate of lime, which may be deposited in permanently 

 disconnected spicula in the walls of the body ; or the spicula 

 may run into one another, and form solid networks, or dense 

 plates, of calcareous matter. When the latter is the case, the 

 calcareous deposit may invade the base and lateral walls of 

 the body of the Actinozoon, thus giving rise to a simple cup, 

 or theca. The skeleton thus formed, freed of its soft parts, is 

 a " cup-coral," and receives the name of a corallite. 



In a zoanthodeme, the various polyps {cinthozoijids) 

 formed by gemmation may be distinct, or their sev^eral enter- 

 ocoeles may communicate ; in which last case, the common 

 connecting mass of the body, or coenosarc^ may be traversed 

 by a regular system of canals. And, when such compound 



» Partially-digested substances are often found in this axial space, and it is 

 not improbable that it may functionally represent the stomach or the com- 

 mencement of the intestine in higher animals. 



