Earthworms - Locomotion
Above: an earthworm undergoing locomotion and moving toward the top of the page. Each frame
represents a one second time interval from left to right.
Before we can analyse this pattern of locomotion we need to understand how contraction of the
circular and longitudinal muscle layers of the body wall affect the shape of the worm. Circular muscle
fibres run in circles concentric to the circumference of the worm whilst longitudinal muscles run parallel
to the long axis from one end of a segment to another. These muscles operate upon the liquid-filled
body cavity or coelom, which acts as a pressurisable hydrostatic skeleton.
Contraction of the circular muscles makes the worm thinner, but because liquid is essentially
incompressible (and so maintains a constant volume) and the increase in pressure forces the liquid
outwards, stretching the worm, so the worm becomes longer and thinner.
Contraction of the longitudinal fibres shortens the worm, former the coelomic liquid out to the sides
and making the worm fatter.