GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 6l 



when they were under experimentation. The time factor was 

 important. This will be confirmed by the following preliminary 

 experiment A. But for the more detailed study, the shortest 

 possible time was adopted, to avoid fatiguing the animals. 



In all the following tables, the signs indicate positive either to 

 gravity or light by + and negative by . 



III. EXPERIMENTS. 

 i . Preliminary Experiments with Gravity and Light. 



Preliminary Experiment A . A tall beaker full of sea-water was 

 placed upside down in a large dish filled with sea-water. Special 

 care was taken to exclude air bubbles from the beaker. It was 

 then supported on two pieces of glass-tubing about 4 mm. in 

 diameter in the dish in order to let the sea-water within com- 

 municate freely with the outside at the bottom. By means of 

 glass-tubing, sea-water was run into the dish so as to have fresh 

 sea-water always at the bottom, but so as not to let any air 

 bubbles into the beaker. Thirty selected snails were placed 

 under the beaker, and between the two pieces of the glass-tubing, 

 which was not high enough to permit the snails within to crawl 

 out from under the beaker. The whole arrangement was then 

 covered with the box already described, to exclude light. 



About ten minutes later it was found that all the snails had 

 crawled up the vertical (inside) wall of the beaker and gathered 

 at or near its top. The sea-water would be expected to be fresher 

 and better supplied with oxygen at the bottom than at the top. 

 The snails, however, crawled upward just the same. This was 

 repeated several times, but there was no exception to this rule. 



Such results indicate that the upward movements of the snails 

 are not caused by the lack of oxygen but by gravity. This is also 

 fully supported by the fact that the snails crawl upward on moist 

 rocks, or on a moist glass plate, in air where the amount of oxygen 

 does not vary. 



Moreover, in leaving the horizontal bottom of the dish to take 

 positions on the vertical wall of the beaker, the snails must have 

 oriented themselves against the pull of gravity. This orientation 

 of the snails cannot be explained by the mechanical theory of 

 geotropism. It is an active process, one of true response to 



