88 A. J. CARLSON. 



blood being gradually pushed from behind forwards in the toot 

 as there are areas of relaxation on the sole of the foot. The 

 presence of isolated bodies of liquid in the strongly contracted 

 foot is not a sufficient evidence that they are the factors in pro- 

 ducing the waves of locomotion, as similar isolated bodies of 

 liquid are also found in the musculature of the contracted mantle 

 (Aplysia, Pleurobranchced). 



The arrangement of the muscle in the foot and the appearance 

 of the sole of the foot in locomotion are so similar in all gastero- 

 pods that the mechanism by which locomotion is effected is in 

 all probability the same in all. Simroth and Jordan missed the 

 true explanation by not taking into account the part played by 

 the musculature of the dorsal and lateral walls of the body 

 cavity. 



The mode of progression of the snail which furnishes the key 

 to the solution is represented in the series of diagrams on page 

 2. Diagram I. represents the side view of the snail during 

 ordinary locomotion. The edges of the foot touches the ground 

 throughout the whole length of the animal, and a continuous 

 trail of mucus marks the path of progression. When the animal 

 changes this gait to that illustrated in diagrams II. to VII. the 

 head is lifted from the ground and while pushed forward by the 

 progression of the rest of the body the neck or successive por- 

 tions of the anterior end is elongated and the diameter dimin- 

 ished as it leaves the contact with the ground. In a few seconds 

 the anterior third of the animal comes to assume the position 

 shown in diagram II. This elongated head end is being held 

 clear of the ground at an angle of 20 to 30 degrees. All this 

 while the animal progresses by means of the part of the foot 

 still in contact with the ground. When the anterior one fourth or 

 one third of the body has attained position II., the head bends 

 down so that the very anterior end of the foot again comes in 

 contact with the ground. The neck and anterior part of the 

 body are bent to form an arch in the manner shown in diagram 

 III. The distance from the ground to the highest point of the 

 curve is from 2-4 mm. The posterior end of the body now ap- 

 pears literally to flow through this arch to the new point of con- 

 tact (b\ that is, as the posterior part of the body moves forward 



