ACTINIA. 123 



number of peripheral cavities by a series of radial mesen- 

 teries or septa. A section through the upper part shows that 

 the peripheral cavities run up all round a central gullet or 

 oesophagus derived from the ectoderm. The coelenteron is 

 lined with endoderm throughout, but digestion appears to 

 be confined to the central gastric cavity, the peripheral 

 cavities being filled with a more or less nutritive fluid. 



Fig. 56. — Transverse Section through Lower Part of a 

 Young Actinian. 



Magnified. (After Hertwig and others.) 



Gonad. 



Gastric Filaments of 

 Directive Mesentery. 



\ 

 Cavity between Directive Mesenteries. 



The free ends of the mesenteries bear numerous gastric 

 filaments which assist ihe processes of digestion. On the 

 walls of the peiipheral cavities are the muscles and the 

 reproductive organs or gonads. 



The muscles consist of (1) a circular or sphincter muscle 

 running round a slight rim outside the tentacles. Con- 

 traction of the circular muscle causes the rim to tighten 

 over the retracted tentacles like the mouth of a bag. (2) 

 The longitudinal muscles which run down one special side 



