PHYLUM PROTOZOA 



53 



stome, and when the animal is retracted into its case, accu- 

 rately closes the mouth of the latter. 



Colonies occur in many of the Infusoria. Some allies of 



Vorticella (Fig. 19, 9) develop highly complex colonies, the 

 slender stalk branching again and again, and each terminal 

 branch ending in a zooid. A remarkable colonial form is 



