|The Earthworm - body wall
The outermost layer of the earthworm body wall is the cuticle - a delicate membrane of fibrils that, along with the mucus that covers the
skin of the worm, helps retain water. Earthworms generally require moist habitats and are in a sense only partially adapted to life on land
as they rapidly desiccate if caught in sunlight. However, there usual habitat of burrowing in soil and possibly crawling about leaf litter at
night retains plenty of moisture and earthworms are superbly adapted to this environment. The cuticle is secreted by the epidermis - a
covering layer of cells, or epithelium. The epidermis consists of tall columnar cells. Beneath the epidermis is a basement layer of
connective tissue, permeated by nerve cell processes, forming a sub-epidermal nerve network or plexus. Inside this layer is a cylinder of
circular muscle - muscle whose fibres run in circular courses around the worm, investing it in a sheath that can contract to make the worm
thinner. Beneath this is another layer of connective tissue and then a cylinder of longitudinal muscle - muscle whose fibres run lengthwise
from anterior to posterior parallel to the long axis of the worm and whose contractions make the worm shorter. Note that the longitudinal
muscles have a peculiar arrangement - muscle cells are arranged in rows either side of connective tissue partitions (dashed lines) to give
a herring-bone effect. Inside the longitudinal muscle is a layer of connective tissue and a layer of flattened epithelial cells (not visible in
this section) that forms the innermost lining of the body wall and the lining of the coelom and is called the peritoneum.
The earthworm body wall is richly supplied by blood. Capillaries (tiny blood vessels) form loops inside the body wall, extending to just
beneath the epidermis. These capillaries help give the worm its red-brown colour and have a respiratory function - oxygen diffuses across
the cuticle and epidermis into the body wall where it is rapidly absorbed by pigments in the blood and carried away to create a diffusion
gradient along which oxygen diffuses into the wall more rapidly. In this way the skin of the worm is behaving rather like the lining of the
lungs of mammals. Carbon dioxide waste gas is also carried in the blood and this diffuses out across the epidermis.
The cuticle consists of alternating layers of fibres that point first one way and then the other, to give this very fine membrane some strength.
These fibres are made of collagen protein. The underlying epidermal cells are attached to the cuticle via short finger-like processes
(microvilli) that contain fibrous proteins that attach the microvillus to an overlying collagen fibre. The outermost surface of the cuticle, the
epicuticle, consists of a layer of microscop[ic ellipsoidal bodies that give off a fibrous network (this probably has a water-proofing function).
Long microvilli (or are these non-motile cilia?) reac right through the cuticle and presumably have a sensory and possibly a secretory
function. The cuticle is perforated by microscopic pores through which muscus-secreting epidermal cells discharge their mucus.
Neighbouring epidermal cells are bound tightly together by interdigitating processes that knit the cells strongly together and proteins form
desmosomes - structures that act like studs or rivets to bolt neighbouring cells together. (Drawn from the micrographs in Coggeshall, 1966)
The diagram below shows a single chaeta and the enclosing chaeta sac and the muscles that operate the chaeta. Students of zoology
often find it tricky to see what various muscles do (the musculature of many animals is extremely complex!) but the key is simply to
remember that muscles work by shortening - they pull and never push. The chaeta protractor muscles protract or extend the chaeta.
When they shorten, the body wall around the chaeta becomes thinner, which you can see would cause the chaeta to extend outwards.
Shortening of the chaeta retractor muscles (when the protractor muscles relax) pulls the chaetae inwards.
The diagram below shows the arrangement of the body wall muscles. The arciform muscles alter the diameter of the seminal grooves and
so move sperm along the grooves by peristalsis. Note the feathery herring-bone effect of the longitudinal muscle blocks. This arrangement
presumably packs more muscle into a given cross-sectional area for increased strength. There is a suggestion in some literature that there
is a second pair of grooves at the ends of the arciform muscles opposite the seminal groove, though most accounts do not mention this, so
perhaps it is very shallow and hard to see. Whether or not this groove serves any function other than anchoring the arciform muscles is
The diagram below shows the detailed structure of the epithelium (drawn and simplified from the micrographs in Coggeshall, 1966). The
epithelium contains many mucus-secreting cells which contain extensive mucus reservoirs inside their cytoplasm, which discharge through
minute pores in the cuticle. The mucus lubricates the worm to facilitate locomotion and burrowing, and also helps prevent the wrom from
drying out and also acts as a renewable barrier - keeping the worm clean from foreign particles and parasites. The dark granules are
presumed to be pigment. BC: blood capillary.
Left: a section through the body wall, including a chaeta. Above: capillaries can
be seen inside the longitudinal muscle in these sections through the body wall.
Click images to enlarge.