MOTION. 445 
Fig. 239. 
we A a Sf; A 
Z. “a é g 
: ] 
F, A 
Leptopus longipes. 
the centre of gravity c backwards in the 
direction of de, but by the elongation of one 
set of legs from f to h, and subsequent re- 
traction towards f, and the simultaneous con- 
traction of the other set from f” to A’, and 
subsequent extension towards f’, the centre c 
will be propelled laterally in the direction of 
ab, perpendicular to de. The lines a 6, de 
represent both the magnitude and direction 
resulting from the two movements of the legs, 
in g f, g’ f’ and h f, h’ f’ respectively, but, 
by changing the position of the legs, they may 
also move obliquely. 
The Macrourous Decapods, as the lobsters, 
are all organized for swimming ; and they have 
accordingly been considered under that section. 
Gasteropoda.—The motions of the Gastero- 
poda are proverbially slow; the situation and 
Structure of the muscular foot enable them to 
traverse surfaces vertically, as well as horizon- 
tally. The centre of gravity is supported within 
the base formed by the disc of the foot, which 
is organised to expand, contract, and curve in 
every direction; also to produce a vacuum, 
_ and to secrete an adhesive fluid for the pur- 
pose of securing the stability of their position 
on surfaces directed at any degree of obliquity, 
or on the ceilings of rooms, or roofs of buildings, 
in opposition to the force of gravity. 
The Limaces, or slugs, and Helices, snails, 
present in progression a crawling or gliding 
motion. When, for instance, the Helix pomatia 
prepares for moving, the head, neck, and foot 
are first protruded from the shell; the foot is 
next extended on the plane of position, with 
the shell raised upon it; the muscular fibres 
of the foot then produce an alternate contraction 
and elongation of the successive segments of 
its disc, commencing posteriorly and proceed- 
ing forwards by a visible undulatory motion. 
During these alternate elongations and contrac- 
tions, the animal glides perceptibly from point 
to point, though so slowly that many hours are 
required to traverse the distance of a few feet. 
As these animals crawl up the vertical planes 
of a glass window, the successive undulations 
of the foot are plainly visible when viewed 
from the opposite side. Miiller considers them 
capable of producing a vacuum at various por- 
tions of the dise of the foot, thus availing 
themselves of atmospheric pressure in addition 
to their adhesive mucilaginous secretions. The 
patella, or limpet, and similar Gasteropods are 
well known to produce between the foot 
and plane of motion, a vacuum so powerful 
that the shell may be broken rather than the 
animal will suffer itself to be detached from 
the surface to which it adheres. The minute 
dimensions of each undulation of the foot 
render the Gasteropods incapable of traversing 
loose ashes or sawdust placed in their path, and 
‘ these means are consequently often employed 
by gardeners to prevent slugs from destroying 
the young and tender plants. 
Cephalopoda.—The locomotive organs of the 
Cephalopodous Molluscs are adapted to serve 
the triple purpose of legs for terrestrial progres- 
sion, arms fur prehension, and oars for swim- 
ming ; the Loligo and Sepia are also furnished 
with fins, which are placed on each side the dorsal 
aspect of the trunk, and are the organs chiefly 
employed when swimming. Their terrestrial 
progression is performed by eight legs, leaving 
the two long tentacule free for prehension. . 
The fleshy legs of the Cephalopods are extremely 
flexible, of various lengths, and capable of 
moving in every direction, whilst the acetabule 
enable them to lay hold of bodies with great 
force. In walking, the head and trunk are 
inverted, the direction of motion is retrograde, 
and they move very slowly on solids (fig. 240). 
Ophidia.—The Ophidian Reptiles are desti- 
tute of organs capable of supporting and carry- 
ing the trunk in progression, and therefore they 
