COELENTERA TA 73 



side, the ventral, there is a special groove (Fig. 51) lined with 

 long cilia, the siphonoglyph. 



The stomodaeum is supported by eight mesenteries ; on the 

 ventral side of each of these are placed the powerful retractor 

 muscles, which draw the polyp swiftly into the corallite at the 

 approach, of danger. The two dorsal mesenteries alone are 

 continued far below the lower edge of the stomodaeum in the 

 form of mesenteric filaments, their edge being much thickened, 

 bilobed, and covered with cilia. The endodermal cells are 

 probably some of them amoeboid, and digestion may be intra- 

 cellular. 



The ova are found attached to the dorsal and dorsolateral 

 mesenteries, immediately below the stomodaeum. 



The tabulae are formed by the mesogloea splitting into 

 two layers : the outer remains attached to the ectoderm, the 

 inner layer with the endoderm ; the latter shrinks away from 

 the outer layer, and then in this contracted condition begins to 

 secrete a fresh layer of spicules, which ultimately stretches 

 across the corallite. Hence the space below each tabula is 

 morphologically a space in the mesogloea. 



The polyps of the TuUpora remain free and distinct one 

 from another ; in other groups of the Ootactinia, however, they 

 may be sunk in a well-developed coenosarc, as in Alcyonium, 

 commonly known as " dead men's fingers," or they may be 

 arranged side by side, their lateral surfaces fusing in the form of 

 a leaf-like plate, as in the Pennatulidae (Fig. 52). In Alcyonium 

 the skeleton is not continuous, but consists of spicules scattered 

 loosely through the coenosarc. The leaf-like plates of Penna- 

 tula are borne on each side of a rachis, this is continued into a 

 stalk free of polyps. Both stalk and rachis are traversed and 

 supported by a long calcified horny rod, secreted by an 

 epithelium whose origin is uncertain. 



Among the Alcyonidae and Pennatulidae the individual 

 zooids are often of two kinds. In Fennatula, for instance, the 

 leaf-like expansions are composed of a single layer of polyps 

 (aufozoids) fused side by side, whilst the zooids (siphonozoids) 

 cover that surface of the rachis on to which the bases of 

 the leaves do not extend, and pass up between the leaves. 

 The zooids differ from the polyps, having no tentacles or 



