XIV ZOANTHARIA MESENTERIES 367 



very great variety. It has been shown that there is frequently 

 a stage in their development during which there are only eight 

 mesenteries. This stage is • usually called the Hdwardsia stage. 

 These eight mesenteries are arranged in bilateral pairs as 

 follows: — One pair is attached to the body-wall and reaches 

 to the dorsal side of the stomodaeum, and is called the pair of 

 dorsal directives ; a corresponding pair attached to the ventral 

 side of the stomodaeum is called the pair of ventral directives. 

 The other two pairs are the lateral mesenteries. To these four 

 pairs are added, at the close of the Edioardsia stage, two additional 

 pairs, making in all twelve mesenteries (cf. Fig. 163). 



These six primary pairs of mesenteries, conveniently called 

 the " protocnemes " by Duerden, may be traced in the develop- 

 ment and recognised in the adult of the majority of Zoantharia. 

 But the number of the mesenteries is usually increased in the 

 later stages by the addition of other mesenteries called the 

 " metacnemes." The metacnemes differ from the protocnemes 

 in that they usually appear in unilateral pairs, that is to say, 

 in pairs of which both members arise on the same side of the 

 stomodaeum, and the number is very variable throughout the 

 group. The space enclosed by a pair of mesenteries is called an 

 " entocoele," and the space between two pairs of mesenteries is 

 called an " ectocoele." 



The twelve protocnemes |are usually complete mesenteries, that 

 is to say, they extend the whole distance from the body-wall to 

 the stomodaeum, while the metacnemes may be complete or in- 

 complete ; in the latter case extending only a part of the distance 

 from the body-wall towards the stomodaeum. 



We find, therefore, in making a general survey of the anatomy 

 of the Zoantharia that there is no general statement to be made, 

 concernino- the number or arrangement of the mesenteries, which 

 holds good for the whole or even for a considerable portion of the 

 genera. 



The bands of retractor muscles are, as in the Alcyonaria, 

 situated on one face only of the mesenteries (except in the 

 Antipathidea and Cerianthidea), but an important character of 

 the Zoantharia is that the muscle bands on the ventral pair 

 of directives are situated on the dorsal faces of these mesenteries, 

 and not on the ventral faces as they are in Alcyonaria. 



In the Edwardsiidea there are only eight complete mesenteries, 



