Unicellular, Microscopic Animals 355 



(from six to eleven) which collect wastes and transport them to the con- 

 tractile vacuole. The latter discharges wastes to the exterior through a 

 pore. After each discharge, a new contractile vacuole is formed. Con- 

 tractions may occur every twenty to thirty seconds, depending upon nu- 

 merous factors. Wastes may also diffuse through the pellicle. An anal 

 spot posterior to the oral groove may be observed as it discharges solid 

 particles. 



Coordination and Sensory Equipment. — Paramecia respond to stimuli 

 as do many other protozoa. When certain stimuli are encountered, a 

 Paramecium may reverse its cilia and swim backward a short distance. 

 Using its posterior end as a pivot, the anterior end then swings about in 

 a circle, testing for the stimulus. When no longer stimulated, the cilia 



Fig. 171.- — Paramecium. Photomicrograph of specimens in conjugation. (Copy- 

 right by General Biological Supply House, Inc., Chicago.) 



move the animal forward again. This is the so-called ''avoiding reac- 

 tion." The optimum temperature for paramecia is slightly less than 

 30° C. When stimulated by heat, a paramecium displays the avoiding 

 reaction, moving toward less heat. Sodium chloride gives a negative 

 chemotaxic reaction, while a weak solution of acetic acid may even attract 

 paramecia. They do not seem to be visibly affected by ordinary light. 

 The longitudinal fibers at the basal granules of the ciHa are probably for 

 the purpose of coordinating the actions of the cilia (Fig. 170). 



Reproduction. — Paramecium divides transversely by binary fission 

 (Fig. 169). Occasionally, this is interrupted by a temporary union of 

 two individuals through a process called conjugation (L. com, together; 

 jugare, to join) (Fig. 171) in which there is a reciprocal fertilization. 



