THE PHYLUM PROTOZOA 



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Pa-ra-nicc i uiTi 



Figure 8.15. Asexual reproduction in several protozoa. For explanations see text. 



the old gullet becomes the gullet of the anterior daughter). The two 

 contractile vacuoles become the posterior vacuoles of the daughters and 

 two new anterior vacuoles are formed. New cilia and basal bodies appear 

 among the old. The micronucleus divides by mitosis. The macronucleus 

 is apparently a compound structure formed by the amalgamation of 

 several sets of chromosomes and merely pulls in half during asexual 

 reproduction with no evidence of mitosis. 



The ameba divides very simply by mitosis; the cytoplasm separates 

 into approximately equal halves. The contractile vacuole passes to one 

 daughter and a new one is formed in the other. 



Sexual Reproduction. Sexual reproduction in free-living protozoa 

 is known in detail for only a few groups: the phytomonads, the foramin- 

 ifera and the ciliates. In numerous other groups fertilization (i.e., the 

 fusion of two gametes) has been observed, but the details of the cycle 

 are not known. \V^herever meiosis has been well studied, the process 

 has been found to be the same as that described in Chapter 6, involving 

 tetrad formation and two divisions. Variations between the groups con- 

 cern the time relations of mitosis, meiosis and fertilization. 



Phytomonads are haploid organisms, each possessing a single set of 

 chromosomes. The zygote never divides by mitosis to produce new cells. 



