THE CORALLIGENA. 



139 



tinozoon, are often different from the rest. Each mesentery 

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



Fio. 29.— Perpendicular section of Actinia hotsatica (after Frey and Leuckart).— a, 

 mouth; &, <,'astric cavity; c, common ciivity. into which the gastric cavity and 

 the intermeseuteric chambers open ; d, iiitermeseuteric ciiambers ; e, thickened 

 free margiu, containing thread-cells of, J\ a mesentery ; g^ reproductive oi-gaii ; A, 

 tentacler 



with a thickened and folded margin ; and these free edges 

 look toward tlie 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, zoanthodemcs. 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 w^alls 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 {anthozoolds) 

 formed by gemmation may be distinct, or their several enter- 

 ocoeles m,ay communicate ; in which last case, the common 

 connecting mass of the body, or eoenosarc, may be traversed 

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



1 Paitially-diiiestoil 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. 



