QO A. J. CARLSON. 



only at three points, the intervening parts being held in the posi- 

 tion of arches clear of the ground. In some instances the foot 

 may be in contact with the ground only at two places, as in dia- 

 gram VI. the tail end may be lifted from the ground before the 

 head bends down to make the next contact. At any one time 

 there will thus be two regions of elongation and decrease in 

 diameter of the body and three regions of shortening and thick- 

 ening of the body and vice versa. 



When moving in this manner the snail does not leave a con- 



o 



tinuous film of mucus along its path as in ordinary locomotion, 

 but a trail like the ones represented in Figs. I and 2 on page 2. 

 These figures are traced from the tracks made by the snails 

 across the table, the surface of which was covered with a thin 

 layer of dust. The arrows indicate the direction of the motion, 

 the dotted areas are the places touched by the foot and hence 

 covered with a film of mucus. Fig. I is a portion of the track 

 of a snail of medium size moving straight ahead at a rapid rate. 

 The areas of contact of the foot with the surface are smaller and 

 the further apart the quicker the progression. Fig. 2 is taken 

 from the trail of a larger snail moving slowly and not in a 

 straight line. The areas of contact are larger and closer to- 

 gether, and may even fuse. 



This mode of locomotion enables the animal to cover the 

 ground much quicker than in the ordinary way, and it was 

 always resorted to by the snail studied (Helix dupetithouarsi} 

 when endeavoring to escape from an enemy. I observed it for 

 the first time after having punctured the apex of the shell pre- 

 paratory to injection of alkaloids into the animal. When re- 

 placed on the table the snail quickly emerged from the shell and 

 started this gallop across the table. But the snail will make 

 use of this mode of locomotion when not in the least injured. 

 For some reason I never succeeded in making a specimen move 

 in this manner across a surface covered with lampblack. If a 

 smoked paper was placed in the path of a galloping snail the 

 animal resorted to the ordinary locomotion on touching it. I 

 have not observed this mode of locomotion in the slugs (Limax, 

 Ariolimax] or in any of the marine gasteropods. 



What is the mechanism of this mode of progression and in 



