MOVEMENTS, ET(J., OE FRESH- WATEli PLANAttlANS. 585 



turned towards the source of the stimulus may be entirely 

 omitted. In this case the head is swept around towards the 

 stimulus without being any further i-aised from the bottom 

 than in the ordinary glide. The duration of the pause 

 immediately following stimuhition is likewise subject to 

 great variation. It may be so diminished as to be imper- 

 ceptible, the worm sweeping the anterior end around through 

 the water without any change in the rate of the glide. The 

 amount of the turn varies with the point of application of the 

 stimulus, the head being turned just far enough to point in 

 the direction from which the stimulus comes. This orieuta- 

 tion, if we may so call it, is generally quite exact. If the 

 stimulus is near the middle line on the edge of the head the 

 turn will be only through a few degrees, while if the auricles 

 are touched it will amount to nearly 90°. This fact indicates 

 the remarkably well-developed co-ordination of the reaction. 

 There is a great deal of variation with regard to what takes 

 place after the turn has been made, and the anterior end is 

 directed towards the stimulus. If the stimulating point is 

 removed immediately after stimulation, so that the animal 

 does not touch it by means of the first reaction, a normal 

 specimen will usually lower the head and continue gliding in 

 the new direction. As has been mentioned, however, in 

 some cases a specimen will continue " feeling" about in the 

 locality for some time. If the stimulating point is held in 

 about its original position after the stimulus luis been given, 

 the first reaction will in most cases bring the head into 

 contact with it. In this event the animal usually moves the 

 tip of the head about over the hair (or other point) for a 

 short time, and then drops back to the bottom and continues 

 gliding. In other cases it will clasp the anterior end about 

 the hair (as in the feeding reaction to be described later), 

 and then in a moment start gliding up over it. When this 

 happens the hair or needle may be moved about in the water 

 or even lifted out of it, and the animal will not let go its 

 hold and drop off. If the needle is held fjuiet, however, the 

 animal will in a short time glide down off it and proceed on 



