II 



ALCYONIUM 



107 



in shape and at its free end has a. slit-like " mouth " surrounded by a 

 circle of eight tentacles each of which is pinnate in form. The mouth 

 opening leads into a flattened tube, the stomodaeum (Fig. 46, st), which 

 hangs down into the coelenteron and opens into the latter at its truncated 

 lower end. This stomodaeum corresponds with the oral cone of the 

 Hydrozoa, but here, instead of projecting outwards, it has become as it 

 were inverted into the interior of the polyp. It is therefore lined with 

 ectoderm. Along one edge the cavity of the stomodaeum^tetes^o 



Fig. 45, 



Alcyonium. x |. A, Colony as found cast up on the shore (/I . rfigttetom) ; B, colony with polyps 

 extended (A. palmatum). 



form the ciliated groove (Fig. 47, A, eg), the cells along the floor of which 

 carry powerful flagella and serve to pump fresh sea-water into the coelen- 

 teron. The stomodaeum does not simply hang freely in the coelenteron 

 but is slung up by eight delicate membranes, the mesenteries, which 

 radiate out from the stomodaeum to the outer wall (Fig. 47, A, M). 

 These mesenteries are covered, as are all surfaces abutting on the coel- 

 ' enteric cavity, by endoderm : each is formed of two layers of endoderm 

 cells with an interposed layer of mesogloea. Along the surface which 

 faces the ciliated groove (often called the " ventral " surface) each 

 mesentery possesses a longitudinal thickening of its endoderm, caused 



