492 Physiology 



ciliates normally tracing left spirals, the cilia beat obliquely backward to 

 the right for forward movement. When the same ciliate swims backward, 

 the cilia beat obliquely forward to the left. Although a given type of 

 spiral is more or less characteristic of a species, five species of Paramecium 

 (40) and four of Frontonia (41) may follow either right or left spirals, 

 although swimming is always more rapid in one direction than in the 

 other. Certain other ciliates swim either in right or in left spirals, but 

 not in both. A right spiral is characteristic of backward swimming in both 

 Parajnecmm and Frontonia, and is independent of the spiral followed in 

 forward locomotion. 



RESPONSES TO STIMULI 



Reactions of Protozoa to different stimuli vary with the species as 

 well as with the nature and intensity of the stimulus. Some species may 

 show no reaction to a stimulus which evokes marked reactions in others. 

 The responses studied most extensively are motor reactions which usually 

 tend to move a sensitive organism toward or away from the source of 

 stimulation with some regularity. The response typically involves the 

 organism as a whole, and the morphological nature of the response de- 

 pends upon and is limited by the structure of the organism. In other 

 words, the response is a stereotyped reaction which depends primarily 

 upon structural features of the species rather than upon the nature of 

 the stimulus. The character of the response seems to be one of "trial and 

 error" (248), rather than an immediate and directly induced orientation 

 to the stimulus as would be required in the usual concept of tropisms. 

 In a typical species of Euglena, which rotates on its long axis and also 

 follows a spiral path in swimming, the reaction to moderate stimulation 

 usually shows the following pattern (248). Following stimulation, the 

 gyrations of the anterior end of the body are suddenly widened, presum- 

 ably by an increase in the transverse thrust of the flagellum, and then 

 normal swimming is resumed in a new spiral path. If the stimulus is still 

 encountered, the reaction is repeated until the organism enters a path in 

 which there is no stimulating effect. If stimulation is intense enough, the 

 flagellate temporarily stops forward movement or may move backward a 

 short distance before turning into a new path. The reaction of a swim- 

 ming ciliate is comparable to that of Euglena. Stimulation causes the 

 organism to swim backward for a short distance, stop, and then swim 

 forward in a new spiral. Or backward swimming may be omitted. If the 

 stimulus is still effective, the characteristic reaction is repeated until the 

 path of the organism eliminates the stimulating effect. Such hypotrichs 

 as Oxytricha often creep about on the substratum without rotation of 

 the body on the long axis. If stimulated while creeping, Oxytricha swims 

 backward, swerves to the right, then swims forward again. The process 

 is repeated until the stimulating effect disappears. In spiral swimming 



