LIVING CELLS (2); CELL MODELS 



UNICELLULAR ORGANISMS 

 Paramecium, a ciliate 



This is one of the commonest fresh-water 

 protozoa. It is found in many pools, where it 

 feeds on bacteria which, in turn, feed on decay- 

 ing vegetation. The ciliates are the most com- 

 plexly organized protists, at the opposite ex- 

 treme from the Rhizopods. Indeed, because 

 they present such a remarkable differentiation 

 of structures and activities, it is hard to remem- 

 ber that these organisms are single-celled. In 

 deference to their unicellularity, we speak of 

 their organs as "organelles." 



Put a small drop of Paramecium culture on a 

 slide, and add about an equal drop of 4% 

 methyl cellulose. Making the medium viscous 

 will slow down the Paramecia so that you can 

 observe them more easily. Carefully cover with 

 a cover slip, supported on small broken pieces 

 of another cover slip, and study under low and 

 high power. Note the minute, whiplike cilia, 

 whose regular, synchronized beat propels the 

 animal through the water. Note the differences 

 in length of cilia in different regions of the body; 

 where are they longest? 



Observe the two clear pulsating structures, 

 the contractile vacuoles, near each end of the 

 body. What functions do they serve? What 

 human organ performs analogous functions? 



Note that the Paramecium is asymmetrical. 

 Note its "mouth," a groove or depression lead- 



ing to a funnel-shaped gullet, at the end of 

 which food vacuoles form. 



Unicellular organisms ordinarily reproduce by 

 fission, whereby a mature cell divides to form 

 two equivalent daughter cells. Each individual 

 could in this way become the origin of an 

 immortal line, perpetually renewing itself by 

 repeated division. If reproduction were perfect, 

 that would do well enough, but as in any com- 

 plicated form of life, aging processes occur. The 

 genetic material of all cells is subject to random 

 changes, called mutations. Aging is in part the 

 result of the accumulation of mutations, which 

 are usually deleterious. 



For this reason there is great advantage in 

 some arrangement that permits the individuals 

 of any stock of organisms to mix their genetic 

 material from time to time, so that out of all 

 possible combinations, individuals emerge that 

 possess particularly advantageous constellations 

 of genetic characters. Sexual reproduction is 

 such a device for regularly mixing genetic ma- 

 terial. Such ciliates as Paramecium, though 

 usually reproducing by fission, at times inter- 

 polate another process, a form of sexual repro- 

 duction, called conjugation. Two mature Para- 

 mecia']o\n together side-to-side, exchange genetic 

 material, separate, and then resume asexual 

 reproduction by fission (see S.P.T., pages 491- 

 492). 



Identical Paramecia ordinarily do not con- 

 jugate. Conjugants, even though they may have 



