Typical Annelids - Segmented Worms

A polychaete bloodworm, Glycera sp. The paddles used for
swimming can be seen clearly in this picture. Such bloodworms have
the red respiratory pigment, haemoglobin, which allows them to get
oxygen from oxygen poor environments such as mud.

A leech in a freshwater stream picture
used permission of Chris Schuster published under Creative
Commons Attribution ShareAlike 2.0 License

Earthworm, Lumbricus sp. The top end is the front, note
that this is rounded while the tail end is noticeably flattened.
The pale ring about 1/4 of the way along is the "clitellum",
this is involved in reproduction and egg case formation.
picture used permission ofMichael Linnenbach published
under GNU Free Documentation License, Version
1.2

Earthworms casts The basic material of many rich soils
around the world. Earthworms swallow the finer parts of the soil
in order to absorb the bacteria and other micro-organisms contained,
they then void the waste. In this way, they sort out the finer parts
of the soil and bring them to the surface. They draw down dead leaves
and other plant material and help to incorporate them into the soil.
picture used permission of Lamiot published
under GNU Free Documentation License, Version
1.2
Cool Annelids - Segmented Worms

Tomopterid worm - Family Tomopteridae Swimming above Davidson
Seamount at 389 meters water depth. picture courtesy
NOAA

A tube dwelling polychaete worm - Feather duster worm Sabellastarte
sp. The feathery structures are radioles that are used to
filter feed in the water, most of the worm is inside a protective
tube than be very hard (calcified), the radioles can be very quickly
brought into the tube if danger threatens. picture
courtesy NOAA

Marine polychaete worm
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Basic Features:
- Vermiform (worm-like!)
- Externally segmented which is reflected on the inside, internal
segments separated by septa (walls) with identical copies of
muscles, nerves and excretory structures.
This division of
the body into similar repeating segments is known as "metamerism"
or "metameric segmentation"
- Each segment has 2 pairs of setae or hairs made of
chitin, these are used for locomotion and may be developed into
paddle-like appendages in aquatic forms
- Hydrostatic (liquid) skeleton comprises of body fluid that
is contained within a segment
- Segments have longitudinal and circular muscles that work
antagonistically (against each other) in locomotions
- Have blood, a simple circulatory system, blood vessels with
valves and a heart
- Reproduction is sexual, many species are hermaphrodite (both
male and female at the same time)
- Members of the group may be land dwelling, in soil or sediments
or aquatic
- Some members show a high degree of encephalization (development
of a head)
- Size from 1mm to up to 3m - typically in the region of 1-20cm
Kingdom - Animalia Phylum - Annelidia
The group includes:
- Earthworms, many different species found almost everywhere
there is soil.
- Polychaetes, mostly marine species, often with extended
setae that form paddles of some kind for swimming. Often very
brightly coloured, can be red, pink or green or some combination.
may be free-living, burrow dwelling or make a hard calcified
tube on rocks or
- Hirudinea, the leeches, best known for their parasitic
bloodsucking habits but most feed on small invertebrates including
earthworms which they swallow whole.
What do annelids eat?
Some annelids are hunters, the prey usually being other small
invertebrates. Some are completely free living, other are tube
dwellers and emerge from the tube to grab suitable prey with their
jaws as they pass by. Many leeches catch their food in this manner
too, often swallowing them whole, even if quite large.
Many annelids are detritivores feeding on sediments and deposits
for what they contain in terms of smaller invertebrates and micro-organisms.
Many tube-dwelling marine worms are like this, they can be seen
at low tide on sand or mud flats, typically there is a depression
in the substrate and then 20-40cm away a pile of extruded mud or
sand that has been processed. Ragworms much used by fishermen feed
in this manner as do terrestrial earthworms which exist almost wherever
there is soil.
There are many such as the feather-duster worms that are filter
feeders, using radioles protruded from their burrows to capture
small planktonic organisms in the water to bring to the mouth. Other
filter feeders may live in tube and siphon water through one entrance
and out again through a second in a similar manner to the rag worms
above.
Some leeches such as the well known medicinal leech are blood
feeders on vertebrates, though relatively few of the group.
They secrete an anti-coagulant called "hirudin" which
stops blood from the wound clotting and makes it easier for the
leech to feed. It also means that wounds made by these leeches may
bleed profusely for several hours after the leech has had its fill
and has dropped off. Medicinal leeches may only need to feed once
or twice a year.
What eats annelids?
Annelids make a good easy to digest protein-rich meal for
many other animals. The lack of a hard skeleton makes them easy
to swallow. They are a major part of the diet for many fish and
for many land animals such as moles, hedgehogs and birds such as
the black bird.
How common are Annelids?
Annelids are often almost invisible in a habitat as they tend
to be burrowing animals when present in largest numbers.
Estimates from marine habitats such as Tampa Bay Florida where
they burrow in the sediment are as high as 13,425 individuals
per square metre.
A typical field has about 1million earthworms per acre,
they quietly and silently plough the earth slowly and relentlessly
over the years. They slowly but surely bury anything on the surface
and are responsible for the layer of soil that enables plants to
grow. Charles Darwin thought they were one of the most important
organisms on the earth, and in no small part responsible for the
gentle rolling landscape of England.
Locomotion
in Annelids

The hydrostatic (contained water) skeleton of annelids is held within
the segments as in A and B above. The segments are separated by
the structures labelled 'septum", there are longitudinal muscles
as in "A" which act against the contained liquid (the
coelomic fluid), these contract and shorten the segments to go to
the state in "B". At this point circular muscles contract
and act against the fluid to go back to "A". A co-ordinated
contraction of these two sets of muscles cause the worm to move
forwards as in C.
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