512 RAYMOND PRAEL. 



activities of veiy few lower orgnnisins. The behaviour of the 

 Protozoa has been quite full}' described and analysed by tlie 

 work of Verworn ('89) and Jennings ('97, '99, '99a, '996, 

 '99c, : 00, :00a, : 007), : OOr, : 01, Jennings and Moore :02). 

 In the earlier work of Verworn 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 

 Infiisoi'ia have been ver}' thoroughlj" worked out by Jennings. 

 In the case of the Infusoi-ia w^e now know exactly the 

 mechanism of the renction to a largo number of stiiiinli. The 

 reactions and general behaviour in the case of two groups of 

 echinoderms are quite thoroughly known from the early work 

 of Preyer {'8G, '87) on the starfish and the recent brilliant 

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

 urchin. These few instances are the only ones in the 

 literature where the movements 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 dealiiig with isolated I'cactions in a variet}' 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 j'et 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, PI an aria, was chosen for 

 several reasons. In the first ])lace, it has come to be a sort of 

 paradigm for work on regenenition, and its biolog}' from that 

 standpoint is alread}' well known. Furthermoi-e, in s(une one 

 or more of its species it is an almost universally distributed 

 form and can alwnys 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 rather complicated. 'J'he importance of possess- 

 ing a detailed knowledge of the activities of a bilaterally 

 syminetrical, free-moving, low organism will be apparent 

 when it is considered that such an oru'anism has never been 



