52 ORGANISMS OF ONE CELL 



then, as with the yeast cell, reproduction is reduced to its lowest 

 terms, simple division. In this process the nucleus of the cell 

 first divides and then the cell body (Fig. 20). 



Encystment. When the environmental conditions become 

 unsuitable for life an Amoeba will secrete about itself a wall or 

 cyst of chitin within which it is protected against adverse 

 conditions such as drought. When conditions are again 

 suitable the cyst wall is ruptured and the organism comes 

 out for a new cycle of growth and reproduction. 



B. FLAGELLATED PROTOZOA. CHILOMONAS AND ALLIED 



FORMS 



When proteid matter, a piece of beef or vegetable, is left in 

 water for a day or so it disintegrates and putrefies under the 

 action of bacteria. In addition to the swarms of these minute 

 bacteria there appear in the water enormous numbers of small 

 flagellated protozoa Chilomonas paramecium. Compared with 

 the smaller bacteria these minute animals are of considerable 

 size but compared to Amoeba proteus they are quite small, 

 having a length of about 25 to 3 o// or microns (i/iooo to 1/800 

 of an inch) . In form they are somewhat like an elongated foot- 

 ball with an obliquely truncated end which we may term the 

 anterior end since this is the end in advance when swimming 

 (Fig. 21). The posterior end is rounded and blunt and has no 

 structural features of importance. The animal moves through 

 the water by means of two parallel flagella which extend out to a 

 distance equal to the total length of the body, the latter being 

 dragged along by the vigorous lashing of the water by these 

 flagella and turning round and round on its long axis as it 

 moves forward. 



It is because of the flagella that Chilomonas is classified as 

 one of the Mastigophora or whip-bearing protozoa. 



The flagella are extremely delicate and impossible to see when 

 the organism is moving but when the cells are killed with iodine 

 they can be made out easily. They are of uniform diameter 

 throughout and enter the body at about the center of the 



