BIOLOGY OF PHYSA 55 



As the foot leaves the solid, it begins to curl along its margin 

 into a more or less closed cylinder. The curling begins at the 

 anterior end of the foot and runs backward as fast as the foot 

 leaves the substratum, so that the foot curls in very much the 

 way it does when one draws a pencil point along its axis as it 

 is spread out on the film. The tip of the foot is last to leave 

 the substratum and the thread is attached to it. As the snail's 

 position changes from the horizontal to the vertical its atti- 

 tude remains to all appearances unchanged except for the 

 curling of the foot lengthwise. This composure is in marked 

 contrast to the behavior of the snail when it is unexpectedly 

 dislodged from the substratum, or when, as it sometimes happens, 

 the flow of mucus which forms the thread is retarded and the 

 snail is brought to a standstill in its upward journey. In the 

 latter case the snail loses its composure and stretches its head 

 upward and downward and twists its body from side to side 

 as though trying to break itself loose. If it is successful, it 

 regains its calm demeanor, but if it fails, it turns about and 

 descends the thread to the substratum. The snail's position 

 in ascending a thread varies; the longitudinal axis of its body 

 may be perpendicular to the substratum and its tentacles straight 

 out in front of the head, or the body may be more or less bent. 

 The snail ascends through the water with a smooth, even glide 

 and with no apparent movement of the body. Indeed, the 

 snail's tranquillity while ascending a thread may make the 

 impression that the mucus is being stretched from the sub- 

 stratum into a fine thread by the upward pull of its body. This, 

 however, is far from the truth. The following observations 

 show that the ascending snail is secreting mucus and leaving 

 it in its track just as it does when on the substratum, except 

 that the mucus is in the form of a thread suspended in the water, 

 (i) One may easily be convinced that the mucus cannot be 

 thus pulled into a thread of so great a length if an attempt 

 is made to stretch a piece of the freshly secreted ribbon 40 or 

 50 times its original length. (2) If the snail comes up on 

 a thread it stops almost instantly when its tentacles touch the 

 film. The same upward pull is exerted on the thread when 

 the snail is thus at rest as before. Yet the thread does not 

 stretch so as to bring the animal perceptibly nearer the film. 

 (3) If a carmine mixture is lodged on the thread as the animal 



