GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 77 



sponded to the time of change from spring to neap tide, during 

 which the specimens out on the rocks were exhibiting a corre- 

 sponding change in phototaxis, for the water did not reach them; 

 their behavior tallied with the description of Mitsukuri, who 

 showed that when desiccated, periwinkles became positively 

 phototactic, and when wet, turned negatively phototactic" (14, 



P- H5)- 1 



Unfortunately, Morse has given no description of his methods 

 of experimentation. But, judging from his statements, he has 

 entirely overlooked the effect of gravity on the animal, as Mit- 

 sukuri did. Was not the supposed "positive phototaxis" "after 

 July 18" really an effect of gravity? Littorina littorea does not, 

 in the writer's opinion, crawl upward "on the rocks" on account 

 of positive heliotropism, but on account of negative geotropism 

 and in spite of negative heliotropism, which is the unmistakable 

 reaction of the animal to light. Negative heliotropism in the 

 animal is demonstrable even at "night," if the experiment is 

 conducted on a horizontal surface, and if there is any source of 

 light present. The writer believes that Morse's statement that 

 "as night approached, they became positively phototactic" is 

 incorrect. What really happens is this: As the light stimulus 

 diminishes, the gravity stimulus becomes preponderant and the 

 animals are controlled by their negative geotropism. 



Frandsen, as already stated, proposes two factors for the de- 

 termination of geotropism in the slug. The second will be con- 

 sidered first. "All the conditions being the same," 1 he says, "it 

 is this factor (the relative proportions of the length of the anterior 

 and the posterior regions of the animal's body) which 'determines 

 whether the head end will be directed up or down.' If the ratio 

 of the length of anterior to posterior region of body is 2 : 3, or 

 more, and the mucus is of good quality and sufficient quantity, 

 the slug will be positively geotactic. If the ratio is 3 : 5, or less, 

 the animal will usually migrate upward, and the nearer the 

 ratio approaches I : 2 the more apt is the slug to respond nega- 

 tively. . . . All slugs have a natural tendency to move towards 

 the earth. This tendency is masked in the animals which are 

 negatively geotactic on a glass plate by the greater pull of gravity 



1 Italics not in the original. 



