XIV ZOANTHARIA MESENTERIES — STOMODAEUM 369 



asexually by fission (see p. 388). The metacnemes are fre- 

 quently formed in regular cycles, and in many genera appear 

 to be constantly some multiple of six (Fig. 163, 5). 



In Aladrepora and Porites ^ the two pairs of directives and 

 two pairs of lateral protocnemes are complete ; the other two 

 pairs of protocnemes are, however, incomplete ; and metacnemes 

 are not developed (Fig. 163, e). 



The stomodaeum is usually a flattened tube extending some 

 distance into the coelenteric cavity and giving support to the 

 inner edges of the complete mesenteries ; in many of the 

 Madreporaria, however, it is oval or circular in outline. In 

 most of the Actiniaria there are deep grooves on the dorsal and 

 ventral sides of the stomodaeum, but in Zoanthidea the groove 

 occurs on the ventral side only and in the Cerianthidea on the 

 dorsal side only. In the Madreporaria these grooves do not occur 

 or are relatively inconspicuous.^ In the Alcyonaria the siphono- 

 glyph exhibits a very marked differentiation of the epithelium 

 (see Fig. 148, p. 334), and the cilia it bears are very long 

 and powerful. It has not been shown that the grooves in 

 the Zoantharia show similar modifications of structure, and they 

 are called by the writers on Zoantharia the sulci. There is no 

 difference in structure, and rarely any difference in size, between 

 the dorsal sulcus and the ventral sulcus in the xictiniaria, and 

 the use of the word — sulculus — for the former is not to be 

 commended. 



The mesenteries bear upon their free edges the mesenteric 

 filaments. These organs are usually more 'complicated in 

 structure than the corresponding organs of the Alcyonaria, and 

 the dorsal pair of filaments is not specialised for respiratory 

 purposes as it is in that group. 



In many genera the mesenteric filaments bear long, thread- 

 like processes — the "acontia" — armed with gland cells and 

 nematocysts which can be protruded from the mouth or pushed 

 through special holes (the " cinclides ") in the body-wall. 



The gonads in the Zoantharia are borne upon the sides of the 

 mesenteries and are usually in the form of long lobed ridges 

 instead of being spherical in form, and situated at th& edges of 

 the mesenteries as they are in the Alcyonaria. 



• Duerden, Mem. Acad. Washington, 3rd Ser. viii. 1902. 

 2 Duerden, I.e. p. 436. 

 VOL. I 2 ^ 



