THE PHYSIOLOGY OF LOCOMOTION IN GASTEROPODS. 9 1 



what way does it differ from the ordinary locomotion ? Referring 

 again to the diagrams on page 2 it is obvious that the series of 

 changes of form and position of the anterior third of the body 

 resulting in the position shown in diagram II. cannot be brought 

 about by the musculature of the foot alone, even if aided by iso- 

 lated bodies of liquid. The entire musculature of this end of the 

 body must be brought into play. If the transverse and the 

 oblique muscular strands contract gradually at successive levels 

 from behind forwards simultaneous with the relaxation of the 

 longitudinal muscle, this part of body would elongate and 

 decrease in diameter just as actually occurs. The elevated posi- 

 tion of the elongated head end is simply due to the greater relax- 

 ation of the longitudinal muscle in the foot than in the dorsum. 

 The pushing forward of the head end is probably aided by the 

 pressure of the viscera. 



The bending down (or in any direction) of the head end is 

 obviously a result of the contraction of the longitudinal muscle 

 in the foot simutaneous with the relaxation of that in the dorsum. 

 If we assume that the posterior half of the elongated head end 

 retains the original contraction of the several muscle systems and 

 that in the anterior half the foot contracts and the dorsum relaxes, 

 we have the downward movement of the head and the formation 

 of the arch. The passage of the middle third of the body 

 through the ascending limb of the arch is simply the continu- 

 ation of the processes of contraction of the transverse and oblique 

 muscle and relaxation of the longitudinal muscle which brought 

 about the elongation and elevation of the head end. The same 

 muscular mechanisms which must act in the latter case suffice to 

 account for the former. In the descending limb of the arch the 

 relation of contraction and relaxation of the systems of muscles 

 is reversed. The transverse and the oblique muscles relax and 

 the longitudinal muscles contract, pulling that part of the body 

 forward and down to point b (diagram II.). Friction is prob- 

 ably sufficient to prevent the head end being pulled backward 

 instead of the body forward by this contraction. The lifting up 

 and subsequent shortening and thickening of the tail end in 

 diagrams VI. and VII. is simply a relatively sudden contraction 

 of the dorsum of this part and subsequent contraction of all the 



