1 86 



THE PROTOZOA 



oesophagus, until they come in contact with the endoplasm at the 

 base of the latter. Here they are readily absorbed by the endoplasm, 

 in which, together with a small amount of water, they are confined 

 in a small gastric vacuole. The vacuole enlarges by the constant 

 addition of new material, until it is caught up in the current of the 

 endoplasm and dragged away. In this improvised " stomach " it is 

 slowly digested, a new drop being formed in the meantime at the 

 mouth opening. If food is abundant, the animal may become filled 

 with these gastric vacuoles. The liquid of the vacuole is, at first, 

 simply water, like the surrounding medium, but gradually becomes 

 acid through osmosis in the plasm, and the digestible substances are 



D 



Fig. 101. Buccal apparatus. [BUTSCHLl.] 



A, B. Nassula aurea Ehr. A. From the ventral side. B. The buccal apparatus strongly 

 magnified. C. Urostyla grandis Ehr. D. Vorticella nebulifera O. F. M. 



slowly dissolved, the residue being cast to the exterior through the 

 anus. 



Unlike the mouth, the anus, as a rule, is a simple opening in the 

 outer wall (Maupas, '83 ; Butschli, '88), although Stein ('67) described 

 an anal tube in certain forms (Nyctotherus). In the Heterotrichida 

 it is sub-terminal in position. In the Hypotrichida it is never ter- 

 minal, but usually dorsal, and toward the left edge. In the Peritri- 

 chida, especially in the Vorticellidae, the anus opens into the vestibule. 



B. CONTRACTILE VACUOLES 



A certain amount of water is taken in with the food through the 

 mouth, and at the same time (as in those cases where the mouth is 



