8O THE BEHAVIOR OF LOWER ORGANISMS. 



in a previous paper (Jennings, 1902, a) , the cilia cause a current coming 

 from the region in front of the organism to pass along the oral surface 

 to the mouth ; in this way the oral surface comes in contact with the 

 chemical before any other part is affected. It is not surprising, there- 

 fore, that the organism should turn toward the opposite (aboral) side. 



In the rotifera the conditions are parallel to those found in the ciliate. 

 The cilia cause a current, which passes to the mouth, on the ventral 

 surface of the body (Fig. 27). The solution thus reaches the ventral 

 surface first, and the reaction is, as might be expected, a turn toward 

 the dorsal side. 



It should be distinctly stated that this reaction method is not universal 

 in rotifers even toward chemical stimuli. In some of the larger species, 

 bearing auricles, or with the ciliary apparatus of a very complex 

 character in other respects, varied reactions may occur, which I hope 

 to analyze in another paper. 



REACTION TO HEAT. 



This was studied in detail only in Anurcea cochlearis. A large 

 number of the rotifers were mounted in a shallow trough formed of a 

 slide, as described on p. 12, and one end of the slide was warmed by 

 means of the apparatus shown in Fig. 5. The reactions were then 

 observed with the Braus-Driiner stereoscopic binocular. 



As soon as a portion of the slide has been warmed above the optimum, 

 the rotifers in this region turn more strongly than usual toward the 

 dorsal side, so that the course followed becomes a very wide spiral and 

 the animals make little progress. If the heat is increased the revolu- 

 tion on the long axis ceases, while the animals swerve still more 

 strongly toward the dorsal side (Fig. 28), so that they swim in circles, 

 the dorsal surface being directed toward the center of the circle. 

 Usually after circling thus" a short time the animals begin again to re- 

 volve on the long axis, and dart forward. The direction of this dart 

 forward seems purely random. If it carries the animal out of the 

 heated region the forward movement is continued and the animal 

 escapes. If it does not carry the animal out of the heated region the 

 circling toward the dorsal side is quickly resumed, followed by another 

 dart forward. This is continued either until the rotifer passes out of 

 the heated region or until it is overcome by the heat. Usually, if it 

 does not escape soon from the heated region the circling becomes 

 more rapid and continuous and is kept up till the animal is destroyed 

 by the heat. 



If one end of the slide is heated and the animal approaches the 

 heated region from the opposite end the reaction is of the same 

 character as that last described. As soon as the region is reached 



