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.
Can you look at the four diagrams above and see which muscle system is contracting? Top: at rest,
(and longitudinals relax) making the worm longer and thinner. Third down: longitudinals contract (and
circulars relax) making the worm shorter and thicker. Bottom: longitudinal muscles contract in the left
half of the worm and circular muscles contract in the right half of the worm.
Annelid worms, such as ragworms, lugworms and earthworms have one important additional feature
that enables them to gain fine control over their hydrostatic skeletons - intersegmental septa. Indeed,
it is these septa which make these worms segmented. Each septa is a circular muscular membrane
that separates adjacent segments like a wall. The detailed structure of a septum is shown below:
Above: an intersegmental septum in the earthworm Lumbricus terrestris. The lines indicate the
orientation of the various muscle fibres that make up this membrane. Holes in the membrane allow the
dorsal vessel, gut and nerve cord and associated vessels to cross the septum. The hole around the
definite foramen (anatomical hole) that does not fit tightly around the nerve cord - the neural foramen.
The septa mechanically isolate each segment, keeping the coelomic fluid contained within each
segment. To do this they must be able to stop fluid crossing. To accomplish this, the neural foramen is
surrounded by circular muscle fibres that form a sphincter. Contraction of these muscle fibres makes the
foramen smaller, closing the seal, whilst relaxing the sphincter opens the foramen and allows coelomic
fluid to pass from one segment to the next. When closed, the seal of the foramen is very tight and the
septum will rupture under high pressure before the seal breaks.
The septa isolate segments, allowing one segment to contract its longitudinal muscles and shorten and
thicken, whilst the adjacent segment can contract its circular muscles and become thinner and longer.
Without these dividing walls, the fluid would simply squirt from one part of the animal to another and
equilibrate the pressure inside the hydrostatic skeleton, making such fine-tuned movements impossible.
The septa do not completely isolate adjacent segments, however, since they are stretchable and buckle
slightly under pressure. The muscle fibres in the septum can minimise this by contracting to keep the
septum taught and fairly flat. However, the septa do damp down pressure changes in the hydrostatic
fluid of one segment over a few adjacent segments. In this way segments can operate more or less
independently without interfering with one another.
Some worms, such as marine sipunculids, have fluid-filled pressurised hydrostatic skeletons but lack
septa and are not particularly segmented. However, they have a proboscis which assists them in
locomotion, so they do not need the fine-tuning characteristic of earthworms. The septa enable the
particular mode of locomotion and burrowing used by earthworms and give the body flexibility in its
movements. This appears to be one of the main advantages of segmentation - to facilitate movement.
Vertebrates inherited their segmentation from some worm-like ancestor. Fish trunks are divided up into
segments of muscle separated by septa that serve as tendons for muscle attachment. This arrangement
makes possible the fast and efficient swimming movements of fish. Humans have similarly inherited this
type of segmentation - the septa become ribs in the thorax and sheets of tissue separate the abdominal
muscles into segmented six-packs! Humans are essentially worms with arms and legs!
Many other types of worms do not have fluid-filled coeloms to act as hydrostatic skeletons, but possess
either spongy meshlike tissue filled with fluid under pressure (nematodes), or pressurised cells packed
together (as in flatworms) which act as hydrostatic skeletons with different mechanical properties.
So, the segments of an earthworm can act more or less independently of one-another, thanks to the
septa. This makes possible such movements as shown in simplified diagrammatic form below
Above: the middle segment of this nine-segmented model worm has contracted its longitudinal muscles,
making it short and thick. If the worm is in its burrow then this might anchor it against the side-walls.
Chaetae assist by protruding from the thickened segment and stick into the side-walls of the burrow,
since they point backwards, they prevent the worm slipping backwards or being pulled from its burrow.
On the surface, chaetae will anchor such a segment against the ground. Behind this temporary anchor,
the longitudinal muscles contract, shortening the segments which are not anchored, pulling them
forwards (to the top of the page). At the front end, contraction of the circular muscles elongate the
segments, pushing them forwards against the anchored segment. The worm has moved forwards a little.
Now, to keep progressing in this way, the anchor has to move backwards - that is the anchored
segment has to detach and become thinner whilst the segment behind it widens and becomes the next
anchor, until the last segment is anchored and the other segments stretch forwards. Next, the anchor
returns to the front of the worm as the rear segments are pulled up. In this way the temporary anchor
continues to move fore to aft along each segment in turn and the worm crawls forwards.
In reality, several adjacent segments will anchor at once. Also, there may be more than one anchor
point moving back along the worm at the same time. This explains the pattern of movement shown at
the top of this page.
The waves of muscular contraction that pass from front to back as each segment alternately shortens
and widens and then elongates, are called peristaltic waves and the worm is said to move by peristalsis.
Mucus secreted by the worm probably assists in locomotion over surfaces by reducing or increasing
friction when necessary (mucus has the odd property of being able to alter from a sticky fluid to anchor
things to a watery lubricant allowing things to slide over one another).
When burrowing, the worm is able to press against the soil with considerable pressure - the septa allow
a high pressure to be built up within a segment by muscle contraction, thus allowing the segment to
thicken or elongate with force and push against the soil. Worms burrow in this way, by pushing the soil
aside, unless the soil is too compacted, in which case they will eat their way through! The high pressure
also allows the all important chaetae to dig in with considerable force. If one end of the worm is
disturbed whilst it is within its burrow, then it will contract, shortening and thickening and press its
chaetae in against the walls of its burrows to try and stop a bird or other animal pulling it out. After all, if
it loses its tail it grows another one, and if it loses its head, then it has a fair chance of growing another
one of those too!
Some earthworms may burrow several metres down into the soil, whilst others live always in the top few
inches or remain among the leaf litter. Deep burrowing forms tend to be larger (like the giant
megascolecids of Australia and the giant microchaetids of South Africa. Some species seldom, if ever,
come to the surface and these may have chaetae dispersed evenly all the way around the
circumference of their bodies, allowing them to grip their burrows well. Lumbricus terrestris is quite a
large earthworm (up to 20 to 30 cm long) and can burrow quite deep (down to one metre) but returns to
the surface at night to mate and to find leafs or other rotting organic matter to pull down into its burrow
to feed on during the day (hence the English name of nightcrawler).
On slippery surfaces the chaetae may not grip well and peristaltic locomotion would see the worm
slipping backwards and not moving very far! To overcome this, earthworms can arch their bodies and
form their anterior ends into suckers which they use to pull themselves along. In this way they are quite
capable of climbing out of glass beakers!
Other earthworms besides Lumbricus:
Although all segmented earthworms use a similar system of locomotion, there are differences in the
structures involved. The septum of the Indian earthworm Pheretima posthuma may contain as many as
68 perforations (68 foramina, compared to one foramen in Lumbricus terrestris) each regulated by its
own sphincter muscle. It would be interesting to know if there are any mechanical reasons behind such