198 



THE PROTOZOA 



branches and trunks, penetrating the entire system, like the coenosarc 

 of a hydroid (Fig. 109). 



The contractile vacuole never becomes so complicated as in some 

 Ciliata, but consists, usually, of a single vesicle, which may be 

 surrounded by a circle of small vacuoles emptying into it. In some 

 cases there is a short excretory duct leading from the vesicle to the 



excretory pore in the 

 membrane, which, as 

 in the Ciliata, is a per- 

 manent opening. 



These animals en- 

 cyst only for protec- 

 tion, never, apparently, 

 for reproduction. As 

 in the Ciliata, the pro- 

 cess consists of the 

 secretion of a chitinous 

 mantle about the cell, 

 the tentacles being 

 withdrawn into the 

 body. 



Reproduction is 

 almost invariably by 

 simple division, which 

 may be either equal or 

 partial (budding). In 

 the simplest cases the 

 upper portion of the 

 cell is constricted off, 

 and moves away from 

 the lower portion, 

 which remains upon its 

 stalk (/W0/>/rrj'rt, Spir- 

 ophtya, Urnu/a, etc.). 



Fig. no. Exogenous budding in Ephelota BtitsMiatta Tshi. The detached part de- 



N, nucleus. velops cilia and, after a 



longer or shorter free- 

 swimming period, settles down, loses its cilia, and secretes a stalk. 

 Partial division, or budding, may be either endogenous or exogenous. 

 The latter is the simpler; an individual prepares as for division, but 

 instead of dividing into two equal portions, a number of papillae 

 appear at the outer surface, each becomes a bud, receiving a portion 

 of the nucleus (Fig. no). Endogenous division arises by the in vagi- 

 nation of such a -budding area, while the walls surrounding it grow 



