186 



COELENTERATA. 



tentacles even to vanishing point and by the enlargement of their terminal 

 openings to large slit-like stomidia* ; also by variations from the type of 

 mesenterial arrangement. 



The Actiniaria comprise the Sea- Anemones ; they live in the sea, attached to 

 rocks or other bodies, or embedded in sand, or sessile, as commensals, on hermit 

 crab shells. 



Section 1. Hexactiniae. 



With paired mesenteries. The mesenteries of each pair are usually provided 

 with longitudinal muscular fibres on those faces which are turned towards one 

 another, except on the two pairs of directive mesenteries, which carry the 

 longitudinal muscles on the faces turned from one another (Fig. 149). Six or 

 more pairs of mesenteries, increasing in multiples of six.t Mouth slit-like, 

 oesophagus usually with two gonidial grooves. 



FIG. 149 a. Diagram of the growth of the mesenteries in Hexactinians (from Korschelt and 

 Heider). A , stage of Manicina areolata, with eight primary mesenteries, in transverse section 

 (after H. V. Wilson) ; 7>, stage of Aulactinia stelloides with twelve primary mesenteries (after 

 McMurrich). The mesenteries are numbered in the order of their appearance, ec ectoderm ; 

 en endoderm ; s supporting lamella ; / mesenterial filaments. 



Sub-tribe 1. STICHODACTYLINAE. Tentacles arranged radially, some or 

 all of the iutramesenterial chambers communicating with more than one 

 tentacle. 



Fam. 1. Corallimorphidae. With a double or multiple corona of tentacles 

 (marginal, principal, and intermediate accessory), more than one tentacle com- 

 municating with each intramesenterial chamber. Tentacles various ; pedal disc 

 present ; gonads on all the septa ; muscular system weak ; sphincter muscle 

 various ; acontia absent. Corallimorphus Moseley, from the deep-sea ; Corynactis 

 Allman ; Capnea Forbes ; Discosoma Leuck. ; Aureliana Gosse ; Rhodactis 



* Recent researches render it probable that in these cases the tentacles have 

 dropped off. 



t There is, however, variation in this character, even within the section 

 Hexactiniae. 



