The Reactions of Plaiiarians to Light 



107 



Rate of Locomotion. As regards rate of locomotion the records 

 of specific differences exhibit a wide range, although not as great 

 as that of the generic differences existing between Dendrocoelum 

 and Phagocata. 



TABLE XXI 



The average rate of locomotion expressed in millimeters per second 



It is interesting to note in this connection that Parker and Bur- 

 nett ('00, p. 385) give the average rate for Planaria gonocephala 

 as 1.08 mm. per sec, while Pearl ('03, p. 546) records for P. macu- 

 lata 1.48 mm. per sec. in the case of a worm 11 mm. in length 

 and 1.23 mm. per sec. for one 6 mm. in length. An average 

 of these two records, that is, 1.355 "i^i^- P^r sec, might per- 

 haps be comparable with the average (1.272) given in Table XXI, 

 since an equal number of large and small worms from each 

 genus formed the basis on which these averages were calcu- 

 lated. 



Time Required to Leave a Unit Circle. If planarians mvari- 

 ably took a straight radial path in going from the center to the 

 circumference of a circle, the time required to leave a unit circle 

 might be used in computing the rate of locomotion. Such a path, 

 however, is not taken. Nevertheless, records of this kind, although 

 untrustworthy for purposes of accurate calculation, furnish a reli- 

 able criterion for the comparison of generic and specific behavior. 

 The differences in behavior in the representatives of three genera 

 and two species are given in Table XXII. Bdelloura failed so 

 frequently to emerge from the circle that it is excluded from the 

 list. Each genus and species was tried an equal number of times 

 in light of three different intensities. 



