A PLANARIAN WORM 79 
with this feature. Can you explain how? As in other flat 
worms, the turbellarians possess no body-cavity, the primitive 
body-cavity being filled secondarily by a peculiar vesicular 
connective tissue called parenchyma. The muscular system con- 
sists of a layer of strong circular and longitudinal muscles just 
beneath the surface of the body and of oblique muscles passing 
through the parenchyma. 
