72 



PROTOZOA 



produce a swerving. As a result of the combination of progression, rota- 

 tion, and swerving, the animal follows a spiral path, the axis of the 

 spiral being the direction of motion. This axis may be a straight line 

 (Fig. 20). Thus the Paramecium, which is asymmetrical, is enabled to 

 pursue a direct course though it is unable to swim directly forward or 

 backward. 



102. Behavior. — Paramecia react to various stimuli in a manner 

 somewhat similar to amebas, though apparently they are not affected 



Fig. 21." — Diagram to illustrate the avoiding reaction given by a Paramecium to a 

 solid object. {Modified from Jennings, "Behavior of the Lower Organisms," by the courtesy of 

 Columbia University Press.) The arrows between the successive positions represented indi- 

 cate the path followed, and those drawn within the outlines of the animal the direction in 

 which the animal is moving. The direction of movement is also indicated by the position 

 of the cilia; no attempt has been made to show rotation by any other structural features. 



by ordinary light. Their positive thigmotropism is seen in the tendency 

 to come to rest against objects in the water. It is stated that for each 

 chemical to which paramecia apparently give a positive reaction there 

 is an optimum concentration in the region of which they tend to accumulate. 

 The Paramecium possesses a temperature optimum at about 26°C. (69°F.), 

 or about the temperature of ordinary pond water in summer, when para- 

 mecia become most abundant. The animal possesses positive rheo- 

 tropism, which leads it to swim against a current, or upstream. 

 Paramecia also seem to respond negatively to gravity, tending to come to 

 the top of the water in a vessel in which they are confined. The electric 

 current affects the beating of the cilia, but this is not a natural stimulus, 

 occurring only in the laboratory. 



One mode of response of the Paramecium is known as the "avoiding 

 reaction, " which takes place in the following manner: When a Paramecium 



