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THE PROTOZOA 



how species may have originated through structural adaptations of 

 the parts (ectoplasm) that are in direct contact with the environment. 

 The mouth parts, which are functionally the beginnings of the diges- 

 tive system, are formed by the invagination of a limited portion of 



Fig. 100. Actinobolus radians St. 



the ectoplasm in a manner analogous to the formation of the stomo- 

 daeum of Metazoa. The peristome is the beginning of the mouth 

 depression, becoming more and more deeply depressed as the mouth 

 region is reached. It is not present in all forms, the mouth in its 

 original position probably being anterior and terminal in the monaxonic 



