THE PHYSIOLOGY OF LOCOMOTION IN GASTEROPODS. 89 



the successive portions form their respective parts of the curve. 

 The space of ground between a and b is not touched by the foot 

 in any region of the body. While the middle third of the body 

 is thus pushed and pulled forward, elongating, diminishing in 

 diameter and bending away from the ground to form the arch 

 a b (diagram IV.), the head end repeats the performance of dia- 

 gram II. The foot at the head end continues in contact with the 

 ground only for a distance of one to one and a half centimeter. 

 When the head and neck bend away from the ground, the neck 

 elongates as before, and we have the anterior third of the body 

 in diagram IV. in a position similar to that in diagram II., the 

 head ready to bend down to make a new contact (c). The next 

 stage is shown in diagram V. The middle third of the body has 

 advanced to the second point of contact (/>) and the arch stage 

 or state of elongation and decrease in diameter is being assumed 

 by the last third. The head end repeats the performance of dia- 



<X' 



- 



, 



-- 





. 



FIG. 2. Same as Fig. I. Portions of the track of a large specimen moving slowly. 

 ]/i natural size. 



gram II. and IV., till in VI. it is again elongated and raised from 

 the ground. The snail has now traversed a distance equal to the 

 length of its own body, so that only the extreme tail end of the 

 foot touches the original contact (a). The tail end is very much 

 reduced in diameter at this stage. As the head bends down to 

 make the fourth contact (d) the tail end is being lifted high from 

 the ground and pulled up to contact b in the manner shown in 

 diagram VII. Further progression is simply a repetition of 

 these phenomena. When the whole body has come into this 

 mode of progression the foot is thus in contact with the ground 



