92 A. J. CARLSON. 



longitudinal muscle strands. By thus taking into account the 

 entire body musculature, their coordinate contraction and relax- 

 ation suffice to account for these changes in form and position 

 without having recourse to any " extensile muskulatur " or series 

 of isolated bodies of liquid in the foot. 



What has this mode of progression in common with the ordi- 

 nary locomotion ? In the chitons the dorsal shells prevent any 

 considerable contraction and elongation of the dorsum. And 

 even in ordinary locomotion of other marine gasteropods as well 

 as of the pulmonates there is no appreciable elongation and 

 shortening of the dorsum corresponding to the waves of locomo- 

 tion on the sole of the foot. Nevertheless the peculiar mode of 

 progression in the snail just described is probably only an exag- 

 gerated form of the ordinary locomotion. During ordinary pro- 

 gression the animal assumes its greatest length and smallest 

 diameter; to account for this we need nothing further than the 

 contraction of the transverse and the oblique muscles of the dorsal 

 and lateral sides of the body. The waves of locomotion in the 

 foot are diminutive representatives of the waves of relaxation and 

 contraction illustrated in the diagrams on page 2. At the 

 areas of relaxation the sole of the foot adheres closely to the 

 ground, and between these points the sole is slightly elevated. 

 Nevertheless a continuous layer of mucus covers the path of 

 progress, as the areas of contact are close together and subse- 

 quent portions of the foot occupying the same area is pulled for- 

 wards a little so as to finally touch the preceding area of contact. 

 There can be little doubt that the area of contact of the foot with 

 the ground in any region serves as a fixed point through friction 

 and acting on this the contraction of the longitudinal muscles of 

 the foot pulls the neighboring portion of the body forwards. 



PHYSIOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO. 



