546 RAYMOND PEARL. 



uoruial glitle^ and then to come around, as it usually would in 

 a sliort time, so that it was headed in the direction of the 

 light. Then the time which it took the worm to glide three 

 centimetres was taken by means of a stop-watch. If the 

 animal started crawling, or abruptly changed its direction, 

 the trial Avas ruled out. The average rate in millimetres per 

 second determined in this way from twenty trials on two 

 individuals is 1'34. This rate is considerably higher than 

 those obtaiued by I'arker and Burnett (: 00) for P. gono- 

 cephala. In that form they found a rate of 1"04 mm, per 

 second in the case of individuals moving toward a hori- 

 zontal light; 1'12 mm. per second when movement was away 

 from a horizontal light, and 1'08 mm. nev second when the 

 animals were moving under a vertical light. There seems to 

 be a well-marked correlation between the size of individual 

 and the rate of gliding', as would be expected on general 

 grounds, and is apparent from merely qualitative observations 

 on the movement. One of the specimens from which obser- 

 vations were taken was 11 mm. long when extended, and its 

 rate of gliding was 1"48 mm. per second; while the other 

 specimen, which was only G mm. long when extended, showed 

 a correspondingly slower rate of l'2-3 mm. per second. The 

 statistics are, of course, very meagre, and are not offered for 

 any other purpose than to give a concrete idea of the approxi- 

 mate rate of the gliding movement. A thorough quantitative 

 study of this matter of the rate of movement in planarians 

 and other related organisms, and of the effect of different 

 agents on the rate, would, I believe, be very interesting, and 

 might lead to valuable results. I hope to be able to make 

 such a study at some future time. 



Lelinert (loc. cit., p. 17) gives a table of the rate of move- 

 ment (presumably in the case of the fresh-water forms the 

 rate of the gliding movement) of several species of flat-worms, 

 lie gives no account of how the data were collected, but his 

 values may be inserted here for the sake of comparison. His 

 rates for Bipalium kewense and B. kewense viridis are — 

 Usual rate, 1 to 1"33 mm. per second; occasional rate, 



