Above: a model of the earthworm Lumbricus terrestris. Anterior to the mouth is the first 'segment' which is a
pseudosegment called the prostomium (lit. infront of the mouth). The prostomium is not a real segment since it does
not form a complete ring but forms the upper lip. The next segment is the first true segment and is called the
peristomium (lit. around the mouth) and forms a ring around the mouth. The peristomium has no chaetae, but the
remaining segments bear 8 small bristles called chaetae or setae. (According to some authours the first four
segments may be achaetiferous, or lacking in chaetae, so this may be a variable characteristic). The eight chaetae of
each segment are arranged in 4 pairs and are sited on the ventral surface - two pairs of ventral chaetae are found
just either side of the midventral line and two pairs are further out in the ventrolateral position (that is just ventral of
the side of the worm). The chaetae in certain segments are slightly longer and sharper and are called genital chaetae
(reported to be sited either on segments 10 to 15, 26 and 32 to 37 or on segments 10, 26, 31 to 38 and sometimes
also segment 25). These genital chaetae are used to grasp and pierce the partner during copulation. Segments 9, 10
and 11 contain whitish ventral thickenings of glandular tissue (not shown) that also assist in grasping the partner and
the longer chaetae are sited just outside of these prominences. Chaetae are some of the segments may be raised up
slightly on a small fleshy protuberance or papilla. Five or six segments, situated from segment 29 to 33 to segments
35 to 37 (different authours report differing positions and this may vary in different populations from different
geographic regions) form the saddle or clitellum - a thickened region about a third-way along the worm which is
thickest dorsally and laterally (where it is hard to discern the individual segments that form the clitellum). The clitellum
may contain two ventral thickenings called tubercles. The anus is borne on the last segment, which is mot a true
segment and is called the pygidium. A mature worm may be 20 to 30 cm in length and possesses about 150 segments.
Apart from the mouth and anus there are numerous openings on the worm's body. Each segment from about segment
5 bears one pair of ventral pores, called nephridiopores, which open into the nephridia, which are the earthworm
equivalent of kidneys, there being one pair of nephridia in each segment. One nephridiopore is sited just outside each
pair of ventral chaetae. Through these pores waste products are excreted. On each of segments 9 and 10, at the
junction of segments 9/10 and 11/11 respectively, is a pair of spermathecal openings. These lead into the four
spermathecae, which receive and store sperm from the partner during copulation. Earthworms are hermaphroditic and
so possess both male and female reproductive systems. These spermathecal openings are more or less in line with
the lateral chaetae (though usually bdrawn slightly ventral to these as in the model above).
On segment 14 is a pair of female gonopores or openings to the female reproductive system from which eggs are
shed during copulation. On segment 15, and in line with the female gonopores just outside the ventral chaetae, are
the male gonopores from which sperm are ejected during copulation. The male gonopores are each flanked by a pair
of obvious fleshy lips that form a raised papilla. Seminal grooves (sperm grooves) are a pair of shallow grooves that
run straight from the male gonopores to the clitellum on the ventral surface. Sperm are transported along these
grooves during copulation by the action of special muscles. Finally, on the dorsal surface are the dorsal pores. These
are very hard to see but there is one per segment, sited along the mid-dorsal line in the grooves between successive
pairs of segments. These open into the fluid-filled body cavity or coelom and coelomic fluid secreted through these
dorsal pores helps to keep the surface of the earthworm moist.
Most earthworms are brown to red in colour. In Lumbricus, the ventral underside is a lighter pinkish or whitish colour
and the dorsal surface a dark brown-red, sometimes almost black, especially in the front portion of the worm. This
colour is due in part to the rich blood supply to the earthworms skin.
The earthworm is a bit of a strange beast in pop culture. We dissect it in early biology class to learn the very basics
of anatomy and few people seem to mind. Dennis Rodman of the Chicago Bulls was affectionately known as "The
Worm" for his ability to wriggle into the middle of a play and come up with the ball. Earthworm Jim, a super-hero
earthworm in a bionic suit, was a hugely successful video game released on many platforms. On the other hand, the
term "worm" is also used negatively. Nobody likes it when someone is trying to "worm" their way into a conversation
and even the best Los Angeles bankruptcy lawyer has probably been unfairly accused of being a worm once or
twice. It's safe to say that in that context, the person is not referring to their lawyer as a necessary and vital member
of the ecosystem who gardeners and farmers alike love because of their ability to make plants flourish.
Worms are much-understudied creatures these days. Most animal groups contain principally worms or wormlike
forms, ranging from microscopic nematodes and rotifers to marine polychaetes meters in length. A study of these
worm phyla will reveal that worms are incredibly diverse structurally and physiologically and they span a large
episode in evolution in which nature experimented with numerous physiologies. All the systems of the animal body,
such as musculature, eyes and circulation can be seen at various stages of advancement often with alternate
solutions to life's problems to those used in the vertebrates. Most, if not all worms are far from the simple tubes with
a gut running through the middle that worms are often reduced to for educational purposes. Certain groups such as
the segmented worms or annelids are especially complex. The systems of some worms rival those of the vertebrates
in complexity. Here we shall begin with the segmented worms or annelids. These include extremely diverse and often
very beautiful marine forms bearing tentacles and other appendage to the less ornate leeches and the humble
earthworms. There are quite a few different families and species of 'earthworm' - a name given to any of the
macroscopic terrestrial worms that are found in soil throughout the world. They range in size from a few millimetres to
giants in Australia and Africa which may be up to 4 metres long! As an adaptation to burrowing, earthworms lack
obvious appendages, but closer inspection reveals the presence of bunches of spines or bristles called chaetae in
most earthworms (some earthworms are roundworms or flatworms but these are seldom referred to as 'earthworms').
These are hard to see without a magnifying glass, but they can be felt - running your finger across a worm from back
to front will reveal a rough surface as you brush against the backward-pointing chaetae. These chaetae enable the
worm to grip and because of these chaetae, most earthworms belong to a group called the oligochaetes.
Here we look inside the worm in more detail to give a basic overview of some anatomical features, although this will be
expanded in considerable detail in the various subsections linked to above. Here we shall simply give a basic overview.
The most obvious feature of earthworms is their segmental nature - they are segmented inside as well as outside.
Inside a vertical sheet, diaphragm or septum (plural septa) separates adjacent segments. Many of the body systems,
such as the nephridia, repeat themselves in each segment. Circulatory and nervous systems, however, are less
segmented and run as a continuous system throughout the worm's body. The dorsal blood vessel is situated above the
gut and is often visible externally through the skin as a dark line down the back of the worm. The body wall consists of
a thin, delicate membrane called the cuticle, which helps waterproof the underlying epithelium or epidermis, which is a
single layer of living cells. Beneath the epithelium is a cylindrical sheath of circular muscle with fibres running around
parallel to the circumference in each segment and beneath this a concentric cylinder of longitudinal muscle with fibres
running from font to back. Contraction of the circular fibres makes a segment thinner and longer, whilst contraction of
the longitudinal muscles makes it shorter and thicker. Inside the worm is the tubular gut which occupies the central
fluid-filled cavity or coelom. Beneath the gut is the ventral nerve cord - a double structure consisting of a pair of nerve
cords fused together and enlarging into a small local brain or ganglion in each segment, which gives off nerves to the
muscles and organs of each segment. This gives the nervous system a segmental appearance, however, the nerve
cords run continuously along the length of the animal, backwards from the brain, passing between the intersegmental
septa through a hole (which can be tightly sealed by a surrounding muscular valve). A ventral blood vessel runs
benetah the gut and a subneural vessel beneath the nerve cord.
Above: a cross-section (transverse section, T.S.) through segment 4. In this region the gut forms the pharynx which
leads into the oesophagus more posterior, followed by the crop, gizzard and intestine. The intestine runs all the way to
Above: a cross-section through segment 10, in the region of the oesophagus and its associated glands or pouches. In
each of segments 7 to 11 there is a pair of 'hearts' or pseudohearts - large contractile blood vessels that connect the
dorsal vessel to the ventral vessel and help pump blood around the worm. These have been partly sectioned above.
Also partly sectioned are the pair of \nephridia - coiled tubes that open through the nephridiopores. The spermatheca
that store sperm received from the partner can also be seen and the seminal vesicles, which store the worm's own
sperm prior to release through the male gonopores. Note that the longitudinal muscles are divided by the chaetae into
nine blocks (two large dorsal blocks, one large ventral block and two sets of three ventrolateral blocks).
Above: a section through the posterior region (posterior to the clitellum). In this region, which makes up about two-thirds of
the worm's length, the gut forms a straight tube, the intestine, with a ridge hanging down inside it along its length - the
typhlosole. Chloragogue tissue (chloragogenous cells) surround the intestine.
These structures will be looked at in more detail.
Below are unlabelled versions of the sections for you to test your memory of earthworm anatomy!
pair of genital papillae on segment 26. The ventral chaetae
of this segment are mounted on these papillae which assist
in gripping the partner during copulation. This is the usual
position of these structures in Lumbricus terrestris, like
many external features they differ in different species. Even
within Lumbricus terrestris slight variants in external
features are reported in the literature, and possibly there
are local geographical variants. Indeed, one study found a
mutation with additional genital pores that was quite
common in a particular geographical area. If you examine a
worm in detail, then try to ascertain what species it is and if
it is Lumbricus terrestris, then why not see how closely it
matches the descriptions given here?