78 Reproduction and Life-Cycles 



develop into a posterior girdle, while the intermediate cilia disappear. 

 The primitive ciliary pattern of Holotrichida is thus recapitulated to 

 some extent during excystment of D. nasutiim (12). 



Excystment of Biirsaria truncatella (Fig. 2. 17) is strikingly different. 

 Cyclosis begins early, and a hyaline area of cytoplasm just beneath the 

 emergence-pore becomes more apparent. After a time, the opercular mem- 



Fig. 2. 17. Excystment in Bursaria truncatella; x200 (after Beers). 

 A. Appearance of a "hyaline cap" in the cytoplasm. B, C. Emergence of 

 the ciliate through the ruptured opercular membrane. D. Young excysted 

 ciliate. E. Older stage with developing peristomial membranelles. Key: 

 b, "bridge" joining endocyst and ectocyst; ec, ectocyst; en, endocyst; h, 

 hyaline cap; o, opercular membrane; p, emergence-pore. 



brane bulges outward, and then breaks suddenly as a column of cytoplasm 

 erupts through the pore. The endoplasm streams into the protruded part 

 of the body and ciliary activity, which now increases, tends to move the 

 body through the pore in repeated thrusts. Emergence is completed, 

 posterior end first, and the immature organism swims away. During the 

 next hour the peristomial groove and membranelles are differentiated, 

 and the adult form is gradually assumed (16). 



