DERIVED ORGANIZATION— TAXONOMIC STRUCTURES 169 



separate an anterior frontal field from the remainder of the body, 

 as seems to be the case in Climacostommn virens (Fig. 71, p. 128). 

 With the development of an attaching portion of the body as in 

 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. 86, p. 158). 



Fig. 94. — A, Bursaria truncatella, frontal field deeply insunk; B^Folliculina 

 ampulla, with frontal field drawn out into two flexible arms. (.4, original; B, after 

 Doflein.) 



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

 they may be greatly invaginated as in Bursaria truncatella, or drawn 

 out into two ciliated food-getting arms as in Folliculina ampulla 

 (Fig. 94), or into a tripartite frontal field in Triloba paradoxa, or 

 rolled up in. spiral folds as in Spirochona gemmi para .and Bursalinus 

 synspiralis. 



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 pore, usually in the pos- 



