160 



BIOLOGY OF THE PROTOZOA 



Stentor, and in the interest of feeding, such a frontal field becomes 

 directed upward, reaching its most perfect development in types 

 like Vorticella and its allies (Fig. 74, p. 145). 



Such frontal fields are flat in the various species of Stentor, or 

 they may be greatly invaginated as in Bursaria truncateUa, or 

 drawn out into ciliated food-getting arms as in FoUicuUna ampvUa, 

 or rolled up in spiral folds as in Spirochona gemmipara (Fig. 84). 



Fig. 84. — A, Bursaria truncatdla, frontal field deeply insunk; B, FoUicuUna 

 ampulla, with frontal field drawn out into two flexible arms. {A, original; B, after 

 Doflein.) 



The cytoproct is rarely differentiated as a definite opening in the 

 cortex. In many cases, especially in the flagellate group, the cyto- 

 pharynx and anus are the same. In the majority of ciliates, on the 

 other hand, there is a constant opening or ])ore, usually in the pos- 

 terior region of the body, which is closed and invisible except during 

 the process of defecation (Fig. 30, p. 70). In some forms, notably 



