58 SAKYO KANDA. 



anterior and posterior regions of the animal's body. All the 

 conditions being. the same, it is this factor which 'determines 

 whether the head end will be directed up or down ' ' (3, p. 205). 



The reactions of the marine forms of Littorina littorea, L. rudis, 

 etc., to light and other influences have been studied by Mitsu- 

 kuri (12), Bohn (i), Haseman (4), and Morse (14). Unfortun- 

 ately, however, none of them has taken into consideration the 

 response of the animals to gravity, although the effect of gravity 

 upon Littorina and upon gastropods in general is remarkable 

 and easily observed at the seashore. 



Following the example of Frandsen and of Davenport and 

 Perkins, an attempt was made by the writer with Littorina 

 littorea to determine: (i) "What relation exists between a vari- 

 ation in the pressure of gravity and the precision of orientation?" 

 and (2) What "determines whether the head end will be directed 

 up or down?" 



The experimental work was done in the physiological depart- 

 ment of the Matine Biological Laboratory at Woods Hole, Mass., 

 during the summers of 1912 and 1913. The results obtained in 

 1912 used in this paper are indicated by the date, " 1912." The 

 others were all secured in 1913. 



II. MATERIAL AND METHODS. 



i. Material. The animal used in all the experiments was a 

 marine snail, Littorina littorea, which is numerous about Woods 

 Hole. The snails were collected by the writer in the morning or 

 afternoon, just before a new series of experiments was carried on, 

 and were kept in a large glass dish in running sea-water during 

 experiments. The size of the animal used was about 1.5 x i.i 

 cm. 1 It was found that this was the more convenient size for 

 experimental purposes, because the bigger, i. e., older, ones re- 

 treated into and remained for a long time in their shells when 

 handled. The younger animals are more active and quicker to 

 respond to stimuli. 



The same individuals could not be used throughout any series 

 of experiments; since their movements became abnormal, due 



1 The laboratory collector, Mr. Gray, told me that the snails which I used were 

 about one year old. 



