GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 



4. The Effects of Light and Surface-film of Sea-water. 



Experiment A. Experiments were conducted in the aquarium 

 already described, which was closely surrounded by a four- fold 

 black cloth, all over the sides and bottom. The snails were 

 placed on the glass plate about 2.5 cm. below the surface film of 

 sea- water. Part of the glass plate extended above the surface. 

 By this arrangement, it was possible at the same time to test 

 Haseman's conclusion that the upward and downward movements 

 of the animals "are not due directly to either geotropism or pho- 

 totaxis, but to the action of the film of water" (4, p. 113). The 

 results are given in Table V. 



TABLE V. 



GEOTROPISM OF SNAILS IN AND OUT OF SEA-WATER IN DIFFUSE DAYLIGHT AT 

 DIFFERENT ANGLES OF INCLINATION OF A GLASS PLATE. 



At beginning of experiments each head placed pointing downward. Table shows 

 results after one minute. 



At the angle of 90, none were positively geotropic and only 

 six individuals out of 50 crawled horizontally in sea-water 

 (possibly due to the effect of light). At the angles of 45 and 

 n/4, the number showing " positive geotropism " increased, as 

 would be expected. At the end of one minute, 21 individuals 

 out of 150 (14 at 90 and 7 at 45), as shown in the first column 

 of negative geotropism, crawled upward clear through the surface- 

 film of sea-water into the air. 12 individuals (9 at 90 and 3 at 

 45), as shown in the last column of the same, crawled upward 

 until they reached the surface-film of sea- water, and then 



