ONE-CELLED ANIMALS— PHYLUM PROTOZOA 



105 



Although there are no morphological distinctions between the two 

 paramecia which undergo conjugation, it has been shown that there 

 is some physiological difference which is akin to sex. Just any two 

 paramecia cannot conjugate. There will never be conjugation among 

 the descendants of a single individual — the conjugating animals must be 

 of different mating types. We cannot call one of them male and the 

 other female, however, for in some species of paramecia several dif- 

 ferent mating types have been found and any one can conjugate with 



Two paramecia 

 unite 



Micro nuclei divide 

 macronuclei 

 disintegrate 



Micronuclei divide 

 again. 3 of each 

 4 disintegrate 



Remaining 



micronuclei divide 



into male and 



female portions 



Male portions 

 I move into opposite 

 cells 



Fused nucleus 

 divides 3 times 



giving 8 

 micronuclei 



4 form 



macronuclei, 

 3 disintegrate 



Cell and the 



micronucleus 



divide 



w Exit one 

 'Paramecium 



Animals seperate 

 male and female 

 elements fuse 



Cells and 



micronuclei again 



divide giving 



four eel Is with 



normal nuclei 



Fig. 7.13. Conjugation in Paramecium. These diagrams show the rather complex 

 series of nuclear divisions, disintegrations, and fusions which take place. 



several of the others. Still we can think of these as a beginning of the 

 physiological differences which distinguish sexes in higher forms of 

 animal life and which cause an attraction between those of opposite 

 mating types. 



There are quite a number of other genera of ciliates, including 

 Tetrahymena which we mentioned in the early part of this chapter. There 

 is one species in this class that is parasitic on human beings and it also 

 lives in pigs and chimpanzees. This is Balantidium coll which may be 

 found in the intestine of about 80 per cent of the domestic swine in some 

 localities. It is a large ciliated organism which forms numerous round, 

 thick-walled cysts which occasionally find their way into the human di- 



