PHYLUM PROTOZOA 



25 



ing, the balance is broken and immense numbers make their 

 appearance. This is what happens when an infusion or culture 

 medium, rich in food material, is allowed to stand in the labora- 

 tory to produce a protozoan culture. 



There is a tendency for the forms appearing in laboratory cul- 

 tures to follow a regular though not absolute sequence in the 

 order of their appearance. The first forms to become abundant 

 are frequently minute flagellates (monads). According to the 

 observations of Woodruff these are followed in succession by the 

 Ciliata: Colpoda, Hypotrichates, Paramecium, and Vorticella. 

 Only in very old cultures following the foregoing series did he 

 find Amoeba in relative abundance. 









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Fig. 11. 



Fig. 12. 



Fig. 13. 



Figs. 11-13. — Diagrams to illustrate the methods of asexual reproduction. 

 11, binary fission in Paramecium; 12, multiple fission or spore formation in 

 a protozoan; 13, budding in Acanthocystis. 



Though most species of the free-living, fresh-water Protozoa 

 will reproduce in laboratory cultures, there are some which refuse 

 to do so and though abundant in the plankton are not found in 

 laboratory cultures. Examples are some of the dinoflagellates 

 and ciliates of the family Tintinnidae. 



Reproduction. — Various forms of reproduction are encountered 

 in this group. The one most frequently found is that of binary 

 fission. In this process, the nucleus usually undergoes an indirect 

 division and this is followed by constriction and finally separation 

 of the cytoplasm to form two new individuals (Fig. 11). Com- 

 monly, structures become duplicated in the dividing individual 

 before fission is completed. Fission not uncommonly occurs 

 when individuals are in an inactive or encysted state. A type 

 of multiple fission (Fig. 12) is characteristic of some groups of 

 Sporozoa and Mycetozoa. In this instance, the nucleus under- 

 goes a series of divisions. Each of the nuclear masses thus 

 formed becomes surrounded by a layer of cytoplasm and upon 



