MOVEMENTS, ETO., OE EKESH-WATER PLANARIANS. 547 



rSo mm. per secoucl. This agrees very closely with the rate 

 for P. maculata given above (1"34). His rates for 

 G e o d e s m u s b i 1 i n e a t u s and D e ii d r o c oe 1 u m 1 a c t e u m are 

 considerably slower (Oo — 0"GG mm. per second and 0"75 — 

 1"33 mm. per second respectively). Polycelis tenuis 

 (1*66 — 1*83 mm. per second), Planaria polychroa (2'16 — 

 2"5 mm. per second, exceptionally 3'33 mm. per second), and 

 Mesostomum tetragonum (2"G6 mm. per second) show a 

 markedly faster rate than the forms I have studied. 



Regarding the effect of different agencies on the rate of the 

 gliding movement no special study lias been made, and I can 

 only report a few incidental observations. Such a study 

 should be made by exact quantitative methods, and this I 

 have not had the opportunity to do. What the effect of light 

 on the rate is, it seems to me, impossible to say with entire 

 cei'tainty. Cole and myself have found that light of great 

 intensity (that obtained from a projection lantern with an 

 electric arc as its source of illumination) causes a definite 

 increase in the rate of gliding, but this increase has not been 

 measured. The results of Parker and Burnett do not help us 

 to answer this question of the effect of the intensity of light 

 on gliding, as they are concerned only with the direction of 

 its rays. The well-known phenomenon of " Unterschieds- 

 empfindlichkeit '^ for light which Planaria shows (Loeb, 

 '93, et al.) would indicate that increased light causes 

 increased rapidity of movement. The electric current causes 

 a veiy marked diminution in the rate of gliding in the weakest 

 intensities which affect the organism at all. The effects of 

 chemicals on the rate of gliding are not altogether uniform. 

 Solutions of all chemicals tried with this point in view, when 

 above a certain strength, caused a marked diminution in the 

 rate of the gliding, or else an entire inhibition of it, and the 

 substitution of some other form of movement. The action of 

 weak solutions varied with the different substances. Very 

 weak acids slightly increased the rate. Weak sugar solutions 

 had no observable effect so far as rate of movement was con- 

 ' Unpublished observations. 



