NEGATIVE PHOTOTAXIS ETC. OF Littori II«. 



7 



The foregoing experiments showed that it was easy enough to drive 

 these molluscs up an incline, i. e. from the sea toward the land hut nothing 

 indicated what induced them ever to come down again in state of nature, 

 for the change of the light and dark sides, such as was brought about in 

 Experiment 7, can never take place in the native habitat of these 

 molluscs. After these experiments, therefore, two questions confronted 

 me : — 



(1) What makes these molluscs scatter themselves somewhat 

 uniformly over rocks in state of nature, instead of being 

 amassed together at the highest tide-mark as at the line ab 

 in the Experiment 7 ? 



(2) What induces these molluscs to come down toward the sea 

 after they are once driven up toward the land ? 



The first question was comparatively easy of solution, while the second 

 puzzled me considerably. The following experiments will elucidate 

 these points. 



Experiment 10 : — In Experiment 9, there happened to be two 

 glass-slides stuck on the glass-plate B, and it was noticed that these 

 obstacles stopped for more or less lengths of time some of the molluscs 

 that were going up the incline, and one actually settled there. Acting 

 on this hint I made the following changes in the arrangement. I took the 

 smooth giass-plates A and B (replacing B with a whole one) and poured 

 on them plaster of Paris mixed with coarse sand making an irregular 

 uneven rough surface as nearly resembling the rocks on which these 

 molluscs live as I could make them. These were then placed as before 

 inclined against one side of the square aquarium. About 1 50 individuals 

 were placed at the spot / as before, and a little water was poured until 

 the bases of the glass-plates were just touched. The result was very 

 striking. The molluscs started up the incline as before, but instead of 

 marching straight up to the line ab and of being stopped only by the 

 glass-plate C, they now scattered themselves all over the glass-plates A 

 and B. They availed themselves of the nooks and corners produced by 

 the rough surface and settled down in them. Eig. 5, A shows roughly the 



