242 BEHAVIOR OF THE LOWER ORGANISMS 



In many free-swimming Rotifera the chief methods of movement and 

 reaction are similar even in details to those of the free-swimming infusoria, 

 which we have already described. Like the infusoria, 

 these rotifers swim by means of cilia, revolve on the 

 long axis, and swerve toward one side (usually dor- 

 sal), as they progress. The cilia produce a current 

 passing from in front to the mouth and ventral side, 

 thus allowing the animals to test the conditions in 

 advance. To most effective stimuli these rotifers 

 react, as do the infusoria, by swerving more than 

 usual toward one side, usually the dorsal side. 

 Thus the spiral becomes much wider, and the ani- 

 mals are pointed successively in many different direc- 

 tions and subjected to many different conditions. In 

 time they may thus reach conditions which relieve 

 them of the action of the stimulating agent. There- 

 upon the reaction ceases, so that the animals con- 

 tinue in the direction which has thus been reached. 

 All the general features of the reactions are essen- 

 tially like those of infusoria, so that we need not 

 enter into details. The reactions to mechanical stim- 

 uli, to chemicals, to heat and cold, to light, and to 

 electricity are known to occur in the way just 

 . FlG ' I34 ;~ ? la ~ sketched, in a number of species. Orientation to 



nana, dorsal view. 



After Woodworth. light and to the electric current takes place in the 

 same way as the orientation to light in Euglena and Stentor. It is inter- 

 esting to observe that in the Rotifera, owing to the concentration of the 

 cilia at one end of the animal, there is no such incoherence and lack of 

 coordination in the reaction to the constant electric current, as is 

 found in infusoria. The rotifer (Anurcea cochlearis) becomes oriented 

 with anterior end to the cathode by the same method as in reactions 

 to light and other agents. 



In many rotifers the reaction plan just described forms only one 

 feature of the activities, so that the behavior, taken all together, may be 

 exceedingly complex. There is much opportunity for further study of 

 the reactions of this group. But so far as known, much of the behavior 

 may be expressed as follows: When stimulated, the animals perform 

 continued and varied movements, the variations often taking place in a 

 systematic way. These movements necessarily subject the animals to 

 varied conditions, one of which is finally selected, through the fact that 

 it removes the cause of stimulation. 



Much of the behavior of the flat worm Planaria (Fig. 134), as studied 



