A PLAXARIAX WORM 73 



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. 



