512 RAYMOND PRARL. 



activities of very few lower organisms. The beliaviour of the 

 Protozoa has been quite fully described and analysed by tlie 

 Avork of Verworn ('89) and Jennings ('97, '99, '99a, '99&, 

 '99r, : 00, : OOff, : 00?^, :00c, : 01, Jennings and Moore :02). 

 In the earlier work of Yerworn the general features of most 

 of the reactions of the Protozoa are described, special atten- 

 tion being paid to the rhizopods. The reactions of the 

 Jiifnsoi-ia have been very thoroughly worked out by Jennings. 

 In the case of the In-fusoria we now know exactly the 

 mechanism of the renction to a large number of stimuli. The 

 reactions and general behaviour in the case of two groups of 

 echinoderms are quite thoronghly known from the early work 

 of Preyer {'86, '87) on the starfish and the recent brilliant 

 work of von Uexkiill ('96, '96ff, '99, : 00, :00ft) on the sea- 

 urchin. These few instances are the only ones in the 

 literature where the inovements and reactions of an organism, 

 or group of organisms, have been investigated in any com- 

 prehensive " monographic " way. There is a great body of 

 literature dealing with isolated leactions in a variety of forms, 

 but the thorough investigation of the activities of animals in 

 a way comparable to that in which their morphology has 

 been investigated remains in large degree yet to be done. 



It appeared highly desirable that this sort of knowledge be 

 extended, and it was with this idea in mind that this work 

 was undertaken. The form used, Planaria, was chosen for 

 several reasons. In the first place, it has come to be a sort of 

 paradigm for work (ui regeneration, and its biology from that 

 standpoint is already well known. Furthermore, in some one 

 or more of its species it is an almost universally distributed 

 form and can always be obtained in quantities. Finally, and 

 particularly, it is a representative of an animal type about 

 whose activities we know only the most general facts. It is 

 a symmetrical aquatic organism of low organisation, and its 

 behaviour is rathei' complicated. The importance of possess- 

 ing a detailed knowledge of the .activities of a bilaterally 

 symmc'trical, free-moving, low organism will be apparent 

 when it is considered that such an organism has never been 



