GENERAL ZOOLOGY 



Micronucleus 



Macronucleus 



Trichocysts 

 Cilia 



Oral groove 



Anal spot 



Fig. 8.17. Paramecium: general structure. 

 (Adapted from L. H. Hyman, The Inverte- 

 brates: Protozoa through Ctenopkora, copy- 

 right 1940 by McGraw-Hill Book Co., 

 Inc., printed by permission.) 



blood invaded. It has also been established that chronic infections may per- 

 sist in the tissue cells after the blood has been cleared of parasites by treat- 

 ment; this fact explains the frequent "relapses" often characteristic of the 

 disease and the difficulty of effecting a final cure. 



The Ciliata 



The class Ciliata includes the protozoans in which the body is wholly or 

 partially covered by cilia. Many of the ciliates are complex and highly 

 specialized cells, whose structural complexities far exceed those found at the 

 cellular level in metazoans. A unique feature is the almost universal separa- 

 tion of the nuclear material into two parts, a larger macronucleus and at least 

 one micronucleus, with important differences in function. The genus Para- 

 mecium is representative, and its species have been the subject of many 

 investigations. 



The Paramecium: General Structure. If any forms can be called the 

 omnipresent protozoans of fresh water, they are Paramecium aurelia and P. 

 caudatum. No species of large size occur more commonly in cultures or under 

 a wider range of conditions. Moreover, these species can be easily cultured in 

 the laboratory and are favorable for study. The account to follow deals with 

 P. caudatum, unless otherwise stated. 



The size of the individuals seen in mixed cultures varies greatly. Like 

 other kinds of animals which have been extensively studied, P. caudatum con- 

 sists of many races which breed true among themselves but may differ widely 



248 



