ZOANTHARIA 



189 



foil-shaped in section, and while the functions of digestion and 

 water-circulation are in the Alcyonaria performed by different 

 filaments, here they are performed by different parts of the same fila- 

 ment. Thus, near the stomodaeum, the central part of the filament of 

 a sea anemone or coral is crowded with digestive gland cells and also 

 with nematocysts, while the wings are covered with strongly ciliated 

 epithelium which maintains a current. In the lower part of the me- 

 sentery the filament is exclusively digestive in function : the cells of 



Fig. 141. A, Vertical section through a sea anemone showing primary 

 (right) and secondary (left) mesenteries (dotted) from the endocoelic side. 

 ac. acontia ; ap. aperture in mesentery ; g. gonads ; x. ciliated region, and y. di- 

 gestiv'e region, of mes.f. mesenteric filament ; M. mouth ; or.s. oral sphincter ; 

 retr. longitudinal retractor muscle ; ten. tentacle. B, Transverse section 

 through the ciliated region of mesenteric filament at x. C, Similar section 

 through the digestive region at y. c.ph. phagocytic cells filled with carmine 

 and fish fragments; gl.c. gland cells; nem. nematocysts; z. zooxanthellae. 

 After Stephenson. 



the wings are phagocytic, as is shown by feeding with carmine. From 

 the central part of the filament free threads called acontia are produced 

 in some anemones, which are loaded with nematocysts and may be 

 shot out of the mouth or of special-pores in the body wall when the 

 polyp is stimulated. 



In the corals the skeleton is secreted by the ectoderm, but only by 

 that part of it which forms the basal disc, A flat plate of calcium car- 

 bonate is laid down first of all by the whole of the disc, but almost at 

 once the epithelium is thrown into radial folds and into a circular fold 



