PROTOZOA 31 
mammullated and then band-like, and ultimately undergoes 
fragmentation. The fragments persist some time, and in other 
species, P. caudatum, 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 (Fig. 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 conver, wneiliated, but sometimes 
bears vretractile 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- 
Fic. 24.—ELuplotes patella. After Btitschli. 
. Mouth. 
2. Hypotrichous processes. 
1 
2, 
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 almost lost. | Encyst- 
ment is not uncommon, and one species (Gastrostyla vorax) 
has been kept alive in a hypnocyst for the space of two 
years. 
