738 



SCIENCE. 



LN. S. Vol. XVII. No. 436. 



tigue quickly and periods of rest alternate 

 ■with periods of activity, as in more highly 

 organized animals. (3) There is surprising 

 sensitiveness to mechanical stimidi; it is 

 found that even touching the surface of the 

 water near the animal with a needle point 

 causes a visible reaction. (4) Two types of 

 reaction are given to stimuli: the positive, 

 which is called forth by weak unilateral stim- 

 ulation of the head region, serves to take the 

 animal toward the stimulus (important for 

 the obtaining of food) ; the negative, which 

 results frora strong stimulation of one side 

 of the anterior region of the body, evidently 

 serves to take the animal away from harmful 

 stimuli. (5) Dr. Pearl calls attention to the 

 fact that intensity and not quality of stimula- 

 tion determines which kind of reaction shall 

 be given. In case of all chemicals whose ef- 

 fects were studied it was found that to all 

 solutions above a certain strength the nega- 

 tive reaction was given; to those below, the 

 positive. (6) The reactions to chemicals are 

 practically identical with those to mechanical 

 stimuli. (Y) There is no evidence that plana- 

 rians orient themselves with reference to the 

 lines of diffusing ions of a chemical; instead 

 the reactions are merely repetitions of the 

 positive and negative reactions mentioned. 

 A planarian in the neighborhood of a piece of 

 meat does not turn directly toward the food 

 substance, thus bringing its long axis parallel 

 to the diffusion lines of the substance, but 

 glides along without any evident uniformity 

 of relation to the lines. If it chances to be 

 headed toward the meat when it enters the 

 region of diffusion it obtains the food di- 

 rectly, if not, it continues its forward move- 

 ment until it is stimulated to give the posi- 

 tive reaction. Thus, the forward gliding 

 followed by the positive reaction may be 

 repeated several times before the organism 

 happens to come in contact with the food 

 substance. (8) The ventral surface of plana- 

 rians is strongly positively thigmotactic, 

 whereas the dorsal surface is negatively thig- 

 motactic; hence, when turned over so that the 

 dorsal surface is in contact with a solid, the 

 animal immediately rights itself by an exten- 

 sion of the edge of the body which is in con- 



tact with the solid. The analysis of the 

 righting reaction given by the author is ad- 

 mirable. (9) The reaction to a constant 

 electric current consists of a turning of the 

 head toward the kathode. 



As the author says : " All the normal reac- 

 tions to stimuli are of the nature of reflexes, 

 more or less complex. What the animal will 

 do after a given stimulus, or in a given situa- 

 tion, can be predicted with reasonable cer- 

 tainty. There is, however, some variation in 

 the behaviour, depending on the physiological 

 or tonic condition of the individual at the 

 time of stiraulation. Thus a stimulus suf- 

 ficiently weak to induce the positive reaction 

 in one specimen may cause the negative reac- 

 tion in another; or at different times the same 

 individual may show different reactions — 

 either the positive or negative — to the same 

 stimulus " (p. 703). 



Concerning the psychological position of 

 planaria Dr. Pearl makes some very sane re- 

 marks. His study enables him to say that 

 the reactions of this flat-worm are much more 

 complicated than those of the unicellular 

 organisms as described by Professor Jennings. 

 There is, moreover, a certain amount of vari- 

 ability and adjustment to the demands of a 

 situation. The chief function of the plana- 

 rian brain is the ' preservation of the tonus 

 of the organism,' while the main function of 

 the nervous system as a whole is ' the rapid 

 conduction of impulses.' Dr. Pearl says he 

 does not think we can say whether the worms 

 possess consciousness or not. And he adds: 

 " Any ' objective criterion ' of consciousness 

 does not exist." He might well have said 

 that no such criterion is possible. 



One might with some cause criticise the 

 paper on the ground of undesirable prolixity. 

 The author has everywhere given full descrip- 

 tions of his methods and results, and in addi- 

 tion he frequently gives diagrams to illustrate 

 the reactions. Sometimes these diagrams are 

 quite unnecessary in view of the simplicity 

 of the reaction and the clearness of the verbal 

 description. There is also unnecessary rep- 

 etition throughout the paper. The author 

 has gone to the opposite extreme, in his effort 

 to avoid the omission of significant details. 



