REACTIONS TO STIMULI IN CERTAIN ROTIFERA. 77 



organism usually swerves from the straight line toward the aboral side ; 

 in the Rotatoria it is usually toward the dorsal side. Well-ordered 

 forward progression would therefore not take place, were it not for the 

 revolution on the long axis, converting the circular course into a spiral 

 one. In the Rotifera the revolution on the long axis is, so far as 

 observed, always over to the right. These relations have been brought 

 out in detail in a previous paper by the present author (Jennings, 1901). 

 The spiral path thus followed by most of the free-swimming Roti- 

 fera may be illustrated in Fig. 25, for Anurcea cochlearis Gosse. As 

 will be seen from the figure, the path followed depends upon three 

 factors : (i) the animal continually swerves toward its dorsal side ; (2) 

 it progresses ; (3) it revolves on its long axis. The result of these three 

 factors is the spiral course. In all these relations the rotifer agrees 

 with the infusorian. 



REACTIONS TO STIMULI. 



The most general reaction to a stimulus in such a free-swimming 

 rotifer is an accentuation of one of the factors in this course, namely, 

 the swerving toward the dorsal side. The result is to produce a spiral 

 of much greater width than previously existed. This may often be 

 observed when the vessel containing the rotifers is jarred. It is evi- 

 dent that this method of reaction is fitted to enable the rotifer to avoid 

 a small obstacle lying in its path, that is, in the axis of the spiral. 

 When the animal resumes its former method of swimming the axis of 

 the spiral lies in a new direction ; the course has thus been slightly 

 changed. 



With a stronger stimulus, as when the rotifer strikes against an 

 object lying in its path, the swerving toward the dorsal side may be 

 still more pronounced, while the revolution on the long axis nearly or 

 quite ceases. The result is that the organism swings strongly toward 

 its dorsal side, and when the usual forward swimming is resumed the 

 axis of the spiral lies in a totally new direction (Fig. 26). It thus 

 avoids the obstacle, if the latter is small ; if the first reaction does not 

 avoid the obstacle completely the reaction is repeated until the course 

 is sufficiently altered so that the rotifer no longer strikes against the 

 source of stimulus. In some rotifers the increased swerving toward 

 the dorsal side is preceded by swimming backward a short stretch. 



In all these points the reaction of the rotifer agrees even to details 

 with that of the ciliate infusorian. There is a difference in the fact 

 that the Infusoria are unsymmetrical and cannot therefore be said to 

 swerve toward the dorsal side, as do the prevailingly symmetrical 

 Rotifera. In the Rattulidae, however, we have asymmetry of a char- 

 acter similar to that found in the Infusoria. 



