GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 73 



covered above so that the sunlight penetrated parallel to the glass 

 plate and through the open portion, c. So arranged, the intensity 

 and direction of the rays of the sunlight had to affect the reaction 

 of the snails to gravity. 



Since the animals were negatively heliotropic, it was expected 

 that most of them would crawl downward in this arrangement. 

 Their heads, therefore, were placed upward at beginning, so that 

 definite movements of orientation could be observed. The 



FIG. 3. 



results after one minute were as follows: Of 50 animals in 15 

 trials, 5 or 10 per cent, had crawled upward, all diagonally; 37 

 or 74 per cent, had crawled downward, 22 of them being well 

 oriented and 15 diagonally; 8 or 16 per cent, crawled horizontally 

 and consequently across the lines of direction both of light and 

 of the effective component of gravity. 



The predominance of downward movement was no doubt due 

 to the intensity and direction of the sunlight. This point be- 

 comes clearer in the next experiments, but it is clear that the 

 surface-film has nothing to do with such reactions. 



Experiment D. With the same point in mind, other experi- 

 ments were made in a similar way, but the animals were placed 

 about 1.5 cm. above the surface of the sea-water. In this case 

 29 individuals out of 50 oriented themselves downward, and 

 crawled in that direction through the surface-film of sea-water 

 into it. Three crawled horizontally at the surface-film instead 

 of going into the sea-water. 18 or 36 per cent, crawled upward. 



