AlOVEMIiNTS, ETC., OP FRESH-WATER PLANA RIANS. 650 



effects of ions and the conclusions based on very fine quanti- 

 tative results with chemicals. Two such series of experi- 

 ments as those quoted above from HCl and CuSO^ indicate 

 what would be the worth of the assignment of an absolute 

 value for the concentration of either of these two substances 

 which would produce the positive reaction in planarians. 

 Such instances mig-ht be multiplied, and they serve to bring 

 out the fact, apparently so frequently lost sight of, that what 

 an organism will do when stimulated is quite as much a 

 function of the physiological condition of the organism itself 

 at the time as it is of the stimulus. 



A comparison of these results with those of Yerkes (: 02) 

 on the reactions of Gonionemus is of much interest. This 

 author finds that though there is a well-marked and 

 characteristic food reaction, which is given in response to 

 food substances, whether in solid or liquid form, yet this 

 reaction cannot be induced by other chemicals. It is stated 

 that a number of chemicals were tried in all concentrations 

 for the special purpose of determining whether the food 

 reaction might not depend upon intensity rather than quality 

 of stimulus. This was not found to be the case. We must, 

 then, conclude that Go ni on em us is a stage farther along in 

 its psychic development than is the flat-worm, for the medusa 

 reacts with reference to the quality as well as to the in- 

 tensity and location of the stimulus, while with the flat-worm 

 the intensity and location of the stimulus are by far the most 

 important factors. It is necessary in the case of the flat- 

 worm, to be sure, that there be mechanical and chemical 

 stimuli acting together in order to produce the complex of 

 reflexes forming the complete food reaction, thus indicating- 

 some relation to quality of stimulus. But for the production 

 of what is, in one sense, the most important phase of the re- 

 action, the turning towards the source of stiniulation, the 

 quality of the stimulus is not significant. 



With an understanding of the method of reaction to 

 localised chemical stimuli, a number of interesting special 

 problems present themselves. While it will not be possible 



