524 RAYMOND PEAKL. 



most used, both on account of their abundance and, further- 

 more, because P. dorotocephala is a form particularly 

 favourable for the study of reactions. It is very active, and 

 after being disturbed continues in movement longer than 

 either P. macnlata or P. gonocephala, as has ali-eady 

 been noted by Woodworth CIoc. cit., p. 7). I have found 

 also that it moves faster than either of the other two species. 

 There is a general precisiou and jiositiveness of i-esponse in 

 its behaviour which make it es]iecia]ly favourable for experi- 

 mental work. A large number of experiments have been 

 made with a view to determining whether there was any 

 difference in the i-oactions of these three species, but no 

 essential difference has been found. The form of the reactions 

 is the same in all cases. Whatever differences there are are 

 differences of degree, such as would be conditioned by the 

 relative sluggishness and activity. 



Certain forms of reaction to mechanical stimuli, and to 

 chemical stimuli, are rather more easily induced in P. 

 dorotocephala than in either of the others, yet, as will be 

 shown later, these I'eactions will be given, under the proper 

 conditions, by the other species. This being the case, and 

 since P. dorotocephala was, for reasons noted above, most 

 used in this work, it will be employed throughout the paper 

 as the type form, and it will be understood, when there is no 

 statement of the species, that P. dorotocephala is the form 

 meant. 



No account of the anatomy of these forms will be given 

 here, because it has been very fully treated in other readily 

 accessible papers. The most important papers dealing with 

 the morphology of the fresh-water triclads are those of 

 Jijiina ('84), Lang ('81, '81a), Kennel ('88), Chichkoff ('92), 

 and Woodworth ('91 and '97). 



Besides the species mentioned above, on which the most of 

 the work was done, a number of observations and experi- 

 ments have been made on several other species of triclads 

 and rhabdocoeles. The other triclad most frequentlj' met, 

 and whose reactions have been found to agree closely Avith 



