GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 63 



regard to Preliminary Experiment B is that light is a factor in 

 the behavior of these animals. 



Preliminary Experiment C. According to the "mechanical 

 theory" of geotropism, an animal becomes oriented head up 

 because of its heavier posterior region, or it orients itself with its 

 head down, because of its heavier anterior region or head. An 

 experiment was, therefore, made to determine which region of 

 the snails, the anterior or the posterior, was heavier. This was 

 simple. When an individual was placed on a glass plate in sea- 

 water with its dorsal side down, it came out of the shell, and 

 made an "effort" to "right." It was then carefully dropped and 

 watched as it sank, before it retreated into the shell. Every 

 one of them sank with its anterior region up, as expected. The 

 was done also with those individuals which were positive to 

 gravity at a certain angle of inclination. There was, however, 

 no exception. 



With the same point in mind, empty shells were tested. One 

 of them was fixed by its ventral side on a small square cover glass, 

 with as little glue as possible. At the three corners of the cover 

 glass, fine threads were fastened. The center of gravity was 

 found by suspension to be located somewhere in the posterior 

 region. This was done with a number of shells and the same 

 results were obtained in every case. These results, therefore, 

 agree with the other observations, indicating that the posterior 

 region of the snails is heavier than the anterior region. 



From the above results, the writer is justified in concluding 

 that the center of gravity of the snails is located in the posterior 

 region; and a possible inference from this conclusion is that the 

 posterior region of the snails has a greater specific gravity than 

 the anterior region. This fact and inference may have a bearing 

 in deciding whether the orientation of the animals against gravity 

 is due to purely physical or mechanical pull. This question will 

 be considered throughout all the following experiments. 



2. Experiments to Show the Relation between the Pressure of Gravity 

 and the Precision of Orientation. 



Experiment A. Davenport and Perkins asked and answered 

 by experiment: "What relation exists between a variation in the 



