PROTOZOA 



31 



mammillated and then band -like, and ultimately undergoes 

 fragmentation. The fragments persist some time, and in other 

 species, P. cavdatum, possibly take part in forming the new 

 nucleus. In P. aurelia they ultimately disappear, the majority 

 of them being in all probability extruded. 



Opalina ranarum (Pig. 23) lives in the rectum of a Frog. 

 As it grows, its nucleus divides, until a great number of nuclei 

 are found. The animal then slowly segments, until each 

 portion contains only one or two nuclei; these form a cyst, 

 and in this condition leave the body of the frog. When eaten 

 by a Tadpole, they emerge, grow, and the nucleus again begins 

 to divide. In Opalinopsis this division of the nucleus has 

 been carried further, and it exists in the form of a fine powder 

 scattered through the endoplasm, the particles of which at 

 times coalesce and form a single nucleus again. 



Order IV. Hypotricha. — These Ciliata have a flat ventral 

 surface, completely ciliated, or provided with enlarged muscular 

 cilia. The dorsal surface is convex, unciliated, hut sometimes 

 hears retractile setae. Both mouth and anus are well 

 developed. 



In this group the cuticle is sometimes strongly developed, 

 and forms a protective plate of some thickness; in the Euplo- 



FlG. 24. — Huplotes patella. After Btitsohli. 



1. Mouth. 



2. Hypotriohous processes. 



3. Nucleus. 



4. Cilia of oral groove. 



5. Contractile vacuole. 



tidae this exists on the dorsal surface only. The distinction 

 between endoplasm and ectoplasm is ahnost lost. Encyst- 

 ment is not uncommon, and one species (Gastrostyla vorax) 

 has been kept aHve in a hypnocyst for the space of two 

 years. 



