MODIFI ABILITY OF BEHAVIOR 



171 



In the extended animal the peristomal ciUa are in continual motion. 

 When finely ground India ink or carmine is added to the water, the 

 currents caused by the cilia are seen to be as follows: The mouth of 

 the animal forms the bottom of a vortex, toward which the water above 

 the disk descends from all sides (Fig. 109). Only the particles near 



/ / 



^x il ^ 1 1 ' / /^ k .' 



/ 



Fig. 109. — Sientor raselii, showing the currents caused by the cilia of the peristome. 



the axis of the vortex really strike the disk; those a little to one side 

 shoot by the edges without touching. Particles which reach the disk 

 pass to the left, toward the buccal pouch, following thus a spiral course. 

 Reaching the buccal pouch, they are whirled about within it a few 

 times ; then they either pass into the mouth, at the bottom of the pouch, 

 or they are whirled out over the edge of the pouch, at the mid-ventral 

 notch. In the latter case they usually pass backward along the mid- 

 ventral line of the body (Fig. 109, a), till they reach the edge of the tube. 

 To this they may cling, thus aiding to build up the tube. 



When stimulated, Stentor rceselii may contract into its tube, taking 

 then a short oblong or conical form (Fig. no). Such contractions do 



