34S 
LOCOMOTION — RATE OF PROGRESSION. 
Fig. G12. Fig. 013. 
Schematic figures, showing the arrangement of the extensile 
locomotory muscles and of the varioiiNly distributed contractile 
fibres of the Helt.x foot (after Simrolh). 
Fig. 042. — Plan of the distribution of the muscular fibres as 
seen from above. 
Fig. 043. — Diagrant of the arrangement of the locomotory 
muscles as viewed laterally. 
to the weiglit of the lioily and shell being greater in air tlian in 
water. 
(xastropods generally move with a true gliding motion, the foot-sole 
being continually adherent to the .surface upon which they are crawling 
and not alternately loosened and re-applied thereto. 
The locomotory muscles consists of longitudinal, transverse and 
ohliipie fibres, closely interlaced with the somatic pedal musculature. 
The obliipie contractile fibres shorten the foot and cause the lateral 
bending, the longitudinal fibres being extensile and constituting 
the true locomotory 
muscles ; they are 
stimulated into ac- 
tivity by the action 
of the pedal ganglia 
and nerves, the intri- 
cate plexus of sympa- 
thetic pedal nerves 
which rhythmically 
continue the locomotory muscular action, creating a series of suc- 
cessive and distinctly perceptible waves of contraction and e.xtension, 
flowing over the foot-sole from behind, forwards ; and although the 
continuance of locomotory effort is governed by the rhythmical 
action of the sympathetic system, yet the rate of motion is mani- 
festly shown to be under the control of the animal by their varying 
rates of progress and by the differing numbers of locomotory waves 
passing over the ventral surface of the foot. During the course of 
a brief summer evening’s study of this feature, in a .specimen of 
Helix nemoraUs, at a temperature of 63° Fahr., I noted 50, 42, and 
36 ofthe.se undulatory muscular waves per minute. 
The rate of progression in Gastropods varies in accord with the 
environment, with the size of the animal and in corre.spondence with 
the shape and area of the locomotory disc, those species with long 
and narrow locomotory areas having greater powers of progression 
than tho.se possessing a broad and short foot, and smaller species 
being relatively more active than those of larger size. 
The Limaces, in which the locomotory muscular fibres are restricted 
to the median longitudinal third of the foot-sole, are the most active 
forms, travelling 13 to 14 centimetres per minute or at the rate of a 
mile in about eight days. 
