THE PROTOZOA 



when one race is compared with another. Reproduction, food, and environ- 

 mental factors also influence body size. The cell is spindle-shaped, with the 

 anterior end bluntly rounded and the posterior end more pointed (Fig. 8.17). 

 At one side, a depression, the oral groove, passes diagonally from the anterior 

 end to about the middle of the body, where it ends in a gullet. The body is 

 covered with cilia, which are of uniform length except for those at the 

 posterior end and in the oral groove, which are slightly longer. Within the 

 gullet the cilia are arranged in a special band-like undulating membrane. On 

 the surface of the cell just posterior to the end of the oral groove lies the 

 anal spot, where egestion occurs. 



rhe outermost layer of the cell is a thin, elastic pellicle, which under high 

 magnification shows a geometric pattern (Fig. 8.18) related to the regular 

 distribution of the cilia and trichocysts (Fig. 8.19). Beneath the pellicle is 

 the ectoplasm, from which the cilia and trichocysts originate. The trichocysts, 

 found also in many other ciliates, are structures of problematic function. In 

 the Paramecium they appear to serve as defensive organelles; when stimu- 

 lated, they emit long threads. In other ciliates they aid in the capture of 

 food. 



Fig. 8.18. Paramecium: structure of the 

 pellicle. This is a photomicrograph of a 

 special preparation which demonstrates the 

 arrangement of the plate-like elements in 

 the pellicle. These plates, and their rela- 

 tionships to cilia and trichocysts, are shown 

 in greater detail in Figure 8.19. (Photo- 

 graph courtesy General Biological Supply 

 House, Inc.) 



249 



