THE PHYLUM PROTOZOA 159 



Ciliates are diploid organisms, each possessing a double set of 

 chromosomes. Each zygote becomes an ordinary individual that may 

 give rise to a whole population by mitosis. At sexual reproduction, two 

 individuals of different sexes conjugate (Fig. 8.f7), pressing together 

 their oral surfaces. In each individual, the micronucleus undergoes 

 meiosis. Three of the four meiotic products degenerate (notice that this 

 is comparable to polar body formation in oogenesis), leaving only one 

 viable haploid nucleus. This divides once by mitosis, producing two 

 identical haploid nuclei. One of these from each cell crosses over 

 through the oral region into the other individual and fuses with the 

 haploid nucleus remaining in that cell. Thus, two fertilizations result 

 from each conjugation, and the two new diploid nuclei are identical. 

 The old macronucleus disintegrates and the individuals separate. The 

 diploid nucleus then divides several times and eventually gives rise to 

 a new macronucleus and a new micronucleus. 



Thus, mitosis in the phytomonads is limited to the haploid phase, 

 whereas in the ciliates only a single mitosis occurs in this phase. 



In the foraminifera each generation of haploid animals is fol- 

 lowed by a generation of diploid animals. After fertilization, the zygote 

 develops into a typical foraminiferan, adding chambers as it grows. 

 Throughout this period the nucleus divides by mitosis repeatedly, pro- 

 ducing diploid, multinuclear adults. All of the nuclei subsequently go 

 through meiosis, and the cytoplasm is divided up among the many 

 haploid nuclei. These abandon the parent shell and begin life anew 

 as the haploid generation, growing and adding chambers in much the 

 same manner as the previous generation, except that they remain 

 mononuclear. At maturity, haploid individuals of opposite sex come 

 together in pairs and secrete a membrane around themselves. They then 

 divide rapidly by mitosis, producing large numbers of gametes. The 

 gametes of one individual unite with those of the other to form zygotes 

 that break free from the membrane and begin the diploid generation. 

 Thus, in this group mitosis occurs in both the diploid and haploid 

 phases. 



Sexual phenomena are virtually unknown in such familiar pro- 

 tozoa as the ameba and the euglenas. Apparently some parasitic flagel- 

 lates are diploid, and both haploid and diploid sporozoans have been 

 described. At the present time our knowledge of the place of meiosis in 

 the various cycles is insufficient to warrant general conclusions. 



54. Relationships among the Protozoa 



The flagellates are usually considered to be a basic stock of or- 

 ganisms from which the other protozoa arose. They are thought by 

 some to be the source of higher animals and higher plants as well. As 

 a group they are difficult to exclude from either the plant or the animal 

 kingdom, a problem that has prompted some biologists to erect a third 

 kingdom. Botanists usually claim all of the flagellates in which a photo- 

 synthetic pigment occurs, including closely related forms such as some of 

 the euglenas and dinoflagellates that have lost the pigment. They do 



