548 RAYMOND PEARL. 



cerned. Copper sulphate causes an entire inhibition of the 

 gliding* movement in moderately weak solutions, even when 

 these are not immediately fatal. Weak mechanical stimuli 

 applied at the posterior end of the body cause a slight 

 increase in the rate of gliding, but this is not marked, as any 

 decided stimulus in this region of the body causes the crawl- 

 ing motion to supervene. 



It is probable that the various agents affect the rate merely 

 by causing changes in the general tonus of the animal. There 

 is much evidence to support the view that the rate of gliding 

 of an individual is a direct function of its tonic condition. 

 Thus in the resting condition, which is characterised by a 

 general lowering of the tonus, as will be brought out later, 

 there is little, if any, ciliary movement. Again, after opera- 

 tions Avhicli result in lowering the tonus, the gliding is very 

 slow or entirely absent. 



2. Direction. — The direction of the gliding is, so far as 

 my observations go, always forward. I have never been able 

 to make the animal glide backward. This is in agreement 

 with the finding of Child (: 01) in the case of St idle- 

 st em ma. It, of course, indicates that the effective beat of 

 the cilia cannot be reversed. In the case of the planarians 

 on which this work was principally done, a lateral change 

 of direction of movement is not brought about by the 

 stronger beating of the cilia on one side. In other words, 

 when the animal turns to one side it does so by a muscular 

 bending of the body in that direction, and not by ciliary 

 action. In an undetermined species of triclad, however, I 

 found that the most usual method of turning towards one side 

 was by the stronger beating of the cilia on the opposite side 

 of the body. As an individual was gliding along the bottom 

 of the aquarium dish it would swerve off at an angle to its 

 former course without bending its body in the slightest 

 observable degree. 



b. Crawling Movement. — The second form of loco- 

 motor activity, the crawling movement, is distinctly a 

 muscular movement. It takes place only when the animal 



