REPRODUCTION 



177 



reasons why they are so widely distributed over the surface of the 

 globe. The minute protozoan cysts are easily carried from place to 

 place by wind, attached to soil particles, debris, etc., by the flowing 

 water of rivers or the current in oceans or by insects, birds, other 



Fig. 73. Diagram showing the life cycle of Colpoda cucullus (Kidder and 

 Claff). a-j, normal reproductive activity repeated (j-b) under favorable 

 cultural conditions; k-o, resistant cyst (k-n, nuclear reorganization and 

 chromatin elimination). 



animals to which they become readily attached. The cyst is capable 

 of remaining viable for a long period of time : eight years in Haema- 

 tococcus pluvialis (Reichenow, 1929), four yaers in Spathidium spath- 

 ula and Oxytricha sp. (Dawson and Mitchell, 1929), five years in 

 Colpoda cucullus (Dawson and Hewitt, 1931), 10 years in Didinium 

 nasutum (Beers, 1937), etc. 



