NEGATIVE PHOTOTAXIS ETC. OF LittorilM. 



1 ! 



time, I thought, these molluscs left hy the receding tide and dried hy the 

 sun were mechanically blown down by wind or washed down by rain. 

 But this idea could not be entertained for any length of time, as the 

 following facts show. I marked out with paint areas on rocks around 

 certain counted numbers of individuals, which were also marked with 

 paint. On visiting these areas day after day, it was noticed that some 

 individuals had clearly moved downwards out of the area. Here is an 

 example of such an observation : — 



Station XI. Established Aug. 30 A. M. 20 individuals within the 

 circumscribed area. 



Aug. 31, A. M. No change. 



Sept. 1, A. M. Tide has touched the area. Five individuals 

 have wandered out of the area but none more than 8 cm. 

 3 of these have moved downwards below the urea. 

 Explanation must then be sought elsewhere. Meanwhile I had 

 accidentally discovered the fact that if a stream of water is dropped 

 rather rapidly from above on these molluscs, some of them begin to move 

 down, after the stream is stopped. 



Here is a case. : — 



Experiment 11 (Aug. 29) : — Arrangements as described in Experi- 

 ment 10. In fact they had stood over from that experiment and the in- 

 dividuals were scattered over the artificial plaster-of- Paris rock as in 

 the last phases of that experiment. I now rigged up a siphon arrange- 

 ment by which water could be poured on the molluscs. Dropping of 

 water was begun at 8 : 30 A. M. and continued for 20 minutes. There 

 were about 56 individuals subjected to the action of the stream. When 

 the stream was stopped, 15 individuals moved distinctly downwards. 

 Most of these 15 descended to the water's edge, some rather sinuously, and 

 then wandered horizontally along that edge. 2 made a loop and went up 

 again. 6 unfortunately slid off the glass-plates and went to the vertical 

 back-wall c. About 1'2 of the 56 individuals moved upwards. 



The stream, therefore, induced some to move down toward the light- 



