188 



ZOOLOGY 



SECT. 



are produced into slender threads — the acontia — which may be 

 protruded through the mouth or through special apertures 

 (cinclides) of the body-wall (Fig. 138, A.) 



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

 the two preceding classes. The body- wall (Fig 138) — base, column, 

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

 within, and between them an intermediate layer or mesogloea, 

 which is extremely thick and tough. The gullet (yul.), which, like 

 that of the scyphula, is an in-turned portion of the body- wall, is 

 lined with ectoderm, and its outer surface — i.e. that facing the 

 inter-mesenteric chambers — is endodermal. The mesenteries (mcs.) 

 consist of a supporting plate of mesogloea, covered on both sides by 



mcs 



■nl. ?nes. C 



Fig. 138. — Diagrammatic vertical (A) and transverso (B) sections of a Sea^anemone. The 

 ectoderm is dotted, the endoderm striated, the mesogloea' black, ac. acontium ; en. cinclis ; 

 gul. gullet ; int. mes. c. inter-mesenteric chamber ; met. mesentery ; ines. f. mesenteric 

 filament ; mtk. mouth ; ost. ostium ; %y. pore ; t. tentacle. 



endoderm. The tentacles (t) are hollow out-pushings of the disc, 

 and contain the same layers. 



Muscular System. — Sea-anemones perform various charac- 

 teristic movements : the column may be extended or retracted, the 

 tentacles extended to a considerable length, or drawn back and 

 completely hidden by the upper end of the column being folded 

 over them like the mouth of a bag; the gullet, and even the 

 mesenteries, may be partially everted through the mouth ; and 

 lastly, the whole animal is able, very slowly, to change its position 

 by creeping movements of its base. 



These movements are performed by means of a very well- 

 developed set of muscles. A mesentery examined from the surface 



