130 Herbert Eugene Walter 



('02) in the case of various infusoria as well as Hydra, Hyallela, 

 Clepsine, Stichostema and Physa; by Yerkes ('03) for Goni- 

 onemus and by Carpenter ('05) for Drosophila. 



b Changes in Physiological States Induced by Light 



A variety of stimuli besides light may cause an animal to pass 

 from one physiological state to another. For example, the sense 

 of phototaxis was reversed through mechanical stimulation by 

 Towle ('00) in Cypridopsis and by Holmes ('01, '05b) in Orches- 

 tia and Ranatra. 



The following typical illustration of the manner in which 

 changes from one physiological state to another succeed each other 

 is offered as a basis of comparison with theresponses to light itself, 

 which are about to be described. In the absence of mechanical 

 stimuli a planarian may be in a state of relaxation. Very gentle 

 mechanical stimulation causes the worm to lift its anterior end 

 and move it cautiously about, bringing the animal into a state of 

 slight activity without locomotion. If, now, the mechanical stim- 

 ulus is prolonged or increased in intensity, enough energy is 

 released to put the animal into gliding locomotion, when it may be 

 fairly said to have passed into the state of normal activity. Pro- 

 vided the stimulation is made still more pronounced, the worm 

 can next be forced to forsake gliding for crawling or humping, so 

 passing into the state of violent activity. Further, it is possible by 

 vigorous shaking to throw the worm, temporarily at least, into a 

 condition of inactivity through excessive stimulation, during which 

 the animal would remain quiet, not because it is failing to release 

 any energy, but because it is unable for the time to set free its 

 energy in the form of locomotion. In other words, it is in the 

 state of rigor. Last of all, if mechanical stimulation is repeatedly 

 applied a condition of exhaustion will appear when the worm has 

 no more available energy and so is unable to move at all. 



Effect of Different Intensities. As already pointed out, no inten- 

 sity either of directive or non-directive light was found sufficient 

 to change the condition of normal gliding into crawling. 



