GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 69 



on the plate crawled downward; only 2 oriented negatively and 

 these did not crawl upward; 4 failed to move. As the animals 

 were not compelled to orient themselves in order to move down, 

 the highest percentage of positive geotropism (21 individuals out 

 of 27 or 77 per cent.) was obtained in this case. 



That the character of the surface played a part is evident. 

 But some explanation is necessary. When the cover-box was 

 removed, nine individuals which had crawled down as shown by 

 the mucus were found with the head end of their shells directed 

 vertically, or with slight inclination to the vertical, i. e., their 

 shells mechanically "oriented" upward by the "greater pull of 

 gravity" on the "heavier posterior region." Those individuals 

 probably could not resist the pull of gravity and keep their shells 

 from falling by their weight, while others could. Four individuals 

 crawled downward not quite vertically (on a diagonal line between 

 vertical and horizontal). It may be possible to explain this as 

 a movement which was resultant of the pull of gravity on their 

 shells and on themselves, the resultant of a passive tendency 

 to orientation and an internal response to gravitation. 



From the above observations, one may explain the habitat 

 of the snails, "which usually live on rocky surfaces which lie 

 between the high-tide mark and one foot below the low-tide 

 mark." The limitation of upward movement is due chiefly to 

 the exhaustion of moisture carried by their "foot" from the sea. 

 They seldom, if ever, crawl on dry rocks much higher than high- 

 tide mark, though they do crawl on rocks clear out of sea-water. 

 They seem to stop crawling when the moisture, which they carry 

 with themselves, is exhausted. This agrees with Mitsukuri's 

 and Haseman's observations. 



It is not thinkable that either the dry glass plate, or the dry 

 wooden plate makes it easier for the snails to become oriented 

 and to crawl downward, than the ground-glass plate did. Never- 

 theless on the former surfaces many animals oriented themselves 

 and crawled downward against the resistance of the heavier 

 weight of their posterior region. If this is true, then the upward 

 and downward movements of the snails can not be explained either 

 by the resistance factor or the weight factor or by the both. 



One thing is, of course, possible, that "the quantity and quality 



