i66 



THE INVERTEBRATA 



circulation are in the Alcyonaria performed by different filaments, 

 here they are performed by different parts of the same filament. 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 nemato- 

 cysts, while the wings are covered with strongly ciliated epithelium 

 which maintains a current. In the lower part of the mesentery the 

 filament is exclusively digestive in function : the cells of 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 



nem. 



Fig. 137. 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- 

 gestive region, of Twe^./. mesenteric filament; ikf. 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. 



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 

 which encloses them, and in these are formed vertical walls which 



