REACTIONS TO STIMULI IN CERTAIN ROT1FERA. 



8l 



where the heat acts as an effective stimulus the animal swerves strongly 

 toward the dorsal side, thus beginning to circle, as shown in Fig. 28. 

 If this swerving should continue only till the animal had described a 

 semicircle, then were followed by the forward dart, the animal would 

 of course retrace its original course (or one parallel to it), and would 

 thus escape from the heated region, as happens in the reaction to the 

 electric current (Fig. 29). But the reaction to heat is less precise than 



FIG. 28.* 



this. Usually the animal makes several com- 

 plete circles before darting forward, and the 

 direction in which it darts seems a random 

 one ; sometimes it is toward the heated region, 

 sometimes away from it, sometimes oblique to 

 it. If the path followed leads the animal into 

 the heated region the circling toward the dorsal 

 side, followed by the dart forward, is repeated ; while if the path leads 

 away from the heat no farther reaction is caused and the animal escapes. 

 Thus when a large number of the animals swim toward the heated 

 region a considerable number will be seen a little later to swim away 

 again. But in many cases the dart forward carries the animal still 

 farther into the heated region. These specimens then begin to circle 

 again toward the dorsal side, and if the temperature is high they may 



* FIG. 28. Diagram of a reaction to heat in Anurasa. The unstimulated 

 animal at first advances in the general direction shown by the arrow x, following 

 thus the course a to e. The heat is supposed to be advancing from the direction 

 opposite the arrow x. When the rotifer reaches the point e the heat becomes 

 effective as a stimulus. The animal reacts by turning toward the dorsal side, 

 and continues this so as to describe a complete circle,/", g, //, ', /", etc. ; often it 

 describes such a circle several times. Finally, at some point in the circular 

 course, as g, it resumes the usual spiral course, following thus the path g, /, /. 

 Its original course, shown by the arrow x, has thus been exchanged for a course 

 having the general direction shown by the arrow y. 



