364 L. MURBACH. 



criticism but modifies his former statement to say " a consider- 

 able decrease causes a more gradual cessation of activity." I 

 have no doubt that he has the correct solution of the question, 

 as shown by the experiments above, when he says " the change 

 in intensity of light stimulates the medusa." Tn other words, 

 then, a change in light intensity not only stimulates a resting 

 medusa to move but it may bring a moving individual to rest. 

 This is in accord with well-known facts in the behavior of 

 other animals. 



This leads to another point under discussion, /. e., whether or 

 not " the reactions of a swimming organism are different from 

 those of one at rest." Morse 29 believes they are not and sup- 

 ports his contention with an experiment of letting light fall on 

 one half of a medusa resting in the dark ; then on the half of a 

 swimming individual. In swimming up both turned from the 

 vertical. It is clear that the discrepancy is based on a misappre- 

 hension. To take Morse's own case as an illustration : the light 

 let fall on the resting medusa set it in motion and beyond this, 

 the light produced the same reaction, as it was really in each case 

 falling on a swimming medusa. Therefore Yerkes's statement 

 above is correct. 30 



SURFACE REACTION. 



While there is no doubt that the up-swimming of Gonionennts 

 is directed by gravity as stated by Yerkes, 31 nevertheless light 

 seems to be a more important factor than he holds. Indeed I 

 may say it is a necessary concomitant as may be seen from what 

 follows. That it is not directive Yerkes (page 281) has shown 

 by his experiment of using bottom illumination. The medusa 

 move up to the surface and turn over normally. But casual 

 observation of the upper surface of the water shows that it is 

 sufficiently illuminated from the bottom to allow the medusse to 

 come to the top. So I substituted lateral illumination through 

 an opening near the bottom of the aquarium. The aquarium 



& Jour. of Comp. Neural, Psycho!., 1906, Vol. XVI., Morse, p. 452 ; Yerkes, 

 p. 462. 



29 Amer. Nat., 1907, Vol. XLL, p. 683. 



3(1 Other points are covered in foot-notes 3, 10, 15, pages 356, 357, 359, also page 365. 

 r. Jour. Physio!., 1903, Vol. IX., p. 281. 



