186 



ZOOLOGY 



SECT. 



tertiary mesenteries (mes. 3), which are hardly more than 

 ridges on the inner surface of the body-wall. Thus the entire 

 internal cavity of a Sea-anemone is divisible into three regions : 

 (1) the gullet or stomodceum, communicating with the exterior 

 by the mouth, and opening below into (2) a single main digestive 

 cavity, the stomach or mesenteron, which gives off (3) a number of 

 radially arranged cavities, the inter-mesenteric chambers or metentera. 

 It is obvious that we may compare the gullet and stomach with 

 the similarly named structures in the scyphula-stage of Aurelia, 

 and the mesenteries with the gastric ridges ; indeed, there seems to 

 be little doubt that these structures are severally homologous. A 

 further correspondence is furnished by the presence of an aperture 



B 



FIG. 139. Diagrammatic vertical (A) and transverse (B) sections of a Sea-anemone. The 

 ectoderm is dotted, the endoderm striated, the mesoglcra black, ac. acontium ; en. 

 cinclis ; (/til. gullet ; int. mes. c. inter-mesenteric chamber ; mes. mesentery ; mes. f. 

 mesenteric filament ; mth. mouth ; ost. ostium ; p. pore ; f. tentacle. 



or ostium (ost. 1) in each mesentery, placing the adjacent inter- 

 mesenteric chambers in direct communication with one another : 

 in Tealia a second ostium (ost. 2) is present near the outer edge 

 of the mesentery. Moreover, the free edge of the mesentery 

 below the gullet is produced into a curious twisted cord, the 

 mesenteric filament (mes. /.), answering to a gastric filament of 

 the Scyphozoa. In many Sea-anemones the mesenteric filaments 

 are produced into slender threads the acontia which may be 

 protruded through the mouth or through special apertures 

 (cinclides) of the body-wall (Fig. 139, ^4). 



The general arrangement of the cell-layers is the same as in 

 the two preceding classes. The body- wall (Fig. 139) base, column, 

 and disc consists of a layer of ectoderm outside, one of endoderm 



