PROTOZOA 



43 



definite number of chromosomes, (2) Conjugation is typical of the 

 Ciliata (see p. 23)- (3) Encystment and sporulation are characteris- 

 tic of many forms like Colpoda. In spore-formation there are many 

 simultaneous divisions. 



Encystment, while characteristic of the Sporozoa, is found in ail 

 classes of the Protozoa. Motor organs are withdrawn and the ani- 

 mal forms a test or shell excluding the water, and becoming en- 

 cysted or fixed. 



"In the majority of cases (Minchin, p. 165) an individual in the process 

 of encystment becomes perfectly spherical; occasionally ovoid or pear 

 shaped. Any food particles or foreign bodies are usually ejected or ab- 

 sorbed, contractile vacuoles disappear, all locomotor organs absorbed or 

 cast off. The protoplasm of the organism becomes less fluid and more 

 opaque. Lastly the cyst membrane itself appears around the body. It 

 stands off distinctly from the rest of the body and may vary from a soft 

 slimy or gelatinous coat to a firm membrane, often tough and impervious." 



Protozoa in the encysted state are able to withstand drying, 

 freezing or sun baking. They may be transmitted by winds or 

 birds to a great distance. In parasitic forms, encystment is an 

 adaptation connected with a change from one host to another. 



In the spo?'OZoan parasites y two forms of cysts are distinguishable: 

 I. Full grown forms may produce large resistant cysts, spherical 

 or oval in form. 2. Smallest forms in developmental cycle, the 

 products of multiple fission or sporulation, may secrete around them- 

 selves tough resistant envelopes, within which they may multiply 

 further. In this case, the envelope is the sporocyst and the entire 

 body a spore. " The spores of bacteria are for the most part simply 

 cysts, but are called spores on account of their small size " (Minchin, 

 p. 166). 



The functions of encystment are: i. To protect during adverse 

 conditions. 2. For purpose of digestion after a heavy meal. 3. 

 Reproduction. 4. Reinfection. 



It should perhaps be emphasized at this point that some ciliates 

 (Colpoda) divide on/y after forming a division cyst. 



Regeneration in the Ciliata is a phase of growth. Ciliata with 

 the macronucleus (nutritive nucleus) will live since this nucleus 

 supervises constructive metabolism. Yet Dawson showed that 

 Oxytricha, lacking a micronucleus, would live for 289 generations, 

 from July 10, 1917, to Nov. 17, 1919, without conjugation, reor- 

 ganization (endomixis), or encystment. 



