GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 67 



this serves to indicate that resistance plays a part, particularly 

 in negative geotropism. The resistance theory, however, does 

 not explain why the animals leave the horizontal surface on which 

 they are placed and crawl upward on the vertical wall of a beaker 

 (see preliminary experiment). This fact is unexplained by any 

 mechanical or resistance theory. 



TABLE IV. 



GEOTROPISM OF SNAILS AT THE DIFFERENT ANGLES OF INCLINATION OF A GROUND- 

 GLASS PLATE IN AIR IN TOTAL DARKNESS. 



At beginning of experiments each head pointing downward. Table shows 

 results after one minute. 



Experiment B. A few experiments were made to determine 

 the effect of a dry surface at the angle of 90. The animals ex- 

 perimented on were carefully dried on filter paper. They were 

 placed on a dry smooth glass plate with their heads directed 

 upward. Light was excluded and the animals examined after 

 two minutes. The movements of the animals on the plate were 

 readily seen by the tracks of the mucus secreted by them. 



Of 60 animals used 31 dropped off, 6 or 15 per cent, of the 

 remainder oriented and crawled down, 31 or 79 per cent, crawled 

 upward and one crawled horizontally. The striking fact is the 

 number of individuals that oriented positively and crawled down. 

 This evidence goes to show that the animals tend to become 

 positive on a dry surface, which would evidently serve as a pro- 

 tective reaction when they are. left on rocks by a retreating tide. 



The movements of the animals on the dry plate were regular 



