104 ONE-CELLED ANIMALS— PHYLUM PROTOZOA 



Respiration and excretion are carried on by diffusion through the 

 plasma membrane as in amoeba. The pellicle is highly porous and 

 offers no hindrance to this diffusion. 



In stained specimens of Paramecium caudatum two nuclei can be 

 seen — a larger macronucleus and a smaller micronucleus. We do not 

 know just why there should be such a division of the nucleus in these 

 animals, but it is quite clear that the micronucleus contains the genes of 

 heredity. It can be seen undergoing mitosis when it divides, but the 

 macronucleus does not show this process. 



Reproduction is by transverse fission which is accompanied by 

 a division of each of the nuclei. This may often be seen in the labora- 

 tory since, under favorable conditions, it will occur about every 10 

 or 12 hours and requires about an hour for the completion of the process. 

 Of course such a rate of reproduction cannot continue very long, for all 

 the oceans of the world would be filled with paramecia within a year 

 if it did. Their reproduction slows down as waste products accumulate 

 in the water, as the food becomes exhausted, as temperature becomes 

 unfavorable, or as other unfavorable factors come into play. 



There is also a form of sexual reproduction known as conjugation 

 which takes place in Paramecium. This is best understood in P. 

 caudatum. Two animals come together and unite at their oval grooves. 

 There then follows some interesting changes in the nuclei. The micro- 

 nucleus divides twice and the macronucleus disintegrates. This leaves 

 four micronuclei in each Paramecium. Then three of the micronuclei 

 of each disintegrate, and the one remaining divides into a larger im- 

 motile micronucleus and a somewhat smaller motile micronucleus. 

 The small one of each moves across a connecting protoplasmic bridge 

 and unites with the larger micronucleus of the other animal. Then the 

 two animals separate. So far there has been no actual reproduction, 

 but there will be two divisions immediately following conjugation ac- 

 companied by some nuclear phenomena which are shown in Fig. 7.13. 

 Conjugation is not necessary for the continuation of the species in 

 Paramecium, for one worker isolated a single individual and followed 

 15,000 generations of descendants over a period of 25 years without 

 conjugation ever occurring. It does, however, accomplish an impor- 

 tant objective. In conjugation there is a trading of genes between 

 two individuals. This gives new combinations of hereditary traits 

 which results in a variety in the descendants that would not be possible 

 without such a genie exchange. Variety is important, because through 

 variety there can be a natural selection of the most fit individuals 

 which will continue the propagation of the species. 



