Annelidia - Annelids - Segmented Worms
Annellus - "little ring"

Kingdom - Animalia
    Phylum - Annelidia

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 locomotion
• 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

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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 as bait 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.

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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.

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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 farmers field in the temperate regions 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, they are in no small part responsible for the gentle rolling landscape of England where he studied them.

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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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, ships hulls, wooden structures or flotsam or on the shells of other animals.
• Hirudinea, the leeches, best known for their parasitic bloodsucking habits but most feed on small invertebrates including earthworms which they swallow whole.