76 ANTHOZOA. ZOANTHARIA 



ORDER I. ZOANTHARIA 



In the Zoantharia the tentacles are generally numerous 

 and are never eight in number, as is the case in the 

 Alcyonaria ; occasionally there are six only, but frequently 

 a multiple of six, and they usually form several circles 

 around the mouth. The tentacles are nearly always simple 

 (fig. 22, 1). The mesenteries (fig. 23, a, b, c) are usually 

 numerous also, and form several cycles ; those belonging 

 to the primary cycle are formed first and reach to the 

 stomodseum ; the other cycles (secondary, tertiary, etc.) 

 are successively smaller. The mesenteries are arranged 



d 



e 



Fig. 23. Diagrammatic section of a Zoantharian polyp passing through 

 the stomodasum. a, primary mesenteries ; b, secondary mesenteries; 

 c, tertiary mesenteries ; d, e, primary mesenteries at the ends of the 

 compressed stornodseum. The muscles are indicated by the thicken- 

 ings on the mesenteries. 



in couples (fig. 23) with the longitudinal muscles of each 

 couple facing one another, except in the case of the couples 

 situated at the grooved ends of the stomodseum, where 

 the muscles are turned away from each other (d, e). A 

 skeleton is often present and may be calcareous or horny ; 

 when calcareous it is never composed of spicules but con- 

 sists of aragonite fibres. 



