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 
esophagus derived from the ectoderm, The ccelenteron 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 Hertwic and others.) 
Gonad.: 
Gastric Filaments of 
Directive Mesentery. Gastric 
Cavity. 
Cavity between Directive Mesenteries. 
The free ends of the mesenteries bear numerous gaséric 
jilaments which assist the processes of digestion. On the 
walls of the peripheral cavities are the muscles and the 
reproductive organs or gozads. 
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 
