26 Invcrtebrata. 



surface; or else special areas of it ; sometimes a few of 

 these processes are rigid and act like little feet, or 

 else they are all equal and fine, invisible during their 

 active exercise owing to their rapid rate of motion. 



The outer layer of their body is a firm cuticle 

 which covers a differentiated protoplasmic lamina con- 



FIG. 14. 



Paramcecium aurelia, an infusorian, showing the contractile vesicles (i') 

 cilia and vacuoles (a). 



taining one or more clear spaces or contractile vesicles, 

 which when watched can be seen to expand and con- 

 tract regularly, pulsating like a heart. Within this layer 

 is a more fluid mobile protoplasm containing granules, 

 vacuoles and a pair of singular solid bodies called re- 

 spectively nucleus and nucleolus. 



