The Annelids are the segmented worms; there are about 15 000 species. The body is divided into segments marked externally by circular grooves or annuli or septa. In most segments there is an arrangement of organs and systems; this serial repetition is termed metamerism, and is also found in the Arthropods, and to a certain extent in the Vertebrates. Each segment can serve as a hydrostatic skeleton (see earthworms for more detail), and this increases the efficiency of motion, especially burrowing. Segments are added as the worm grows and matures. The oldest segments are at the head end, and the yougest at the tail end. Annelids (except the leeches) have chitinous bristles called chaetae or setae; these can serve as anchors when burrowing, which is why it is so difficult to pull a worm out of its burrow.

The Annelids are subdivided into three Classes, Polychaeta, Oligochaeta and Hirudinea. It is thought that the Hirudinea diverged from the Oligochaeta.

The Polychaetes are mainly marine and take their name from the numerous chaetae/setae (bristles or hairs) they bear. There are over 10 000 species ranging in size from 1 mm - 3 m. In some mud flats 1 square metre of mud can contain thousands of individuals. Common names are lugworm, blood worm, ragworm and tubeworm.

The chaetae are arranged in paired groups on each parapodia. These are paired appendages, one on either side of almost every segment, they are the fluffy-looking bits on the lugwom Arenicola marina below left and Neanthes virens, below and left. Parapodia are used in locomotion and respiration.

There are a variety of body shapes. Some are sedentary tube or burrow dwelling particle, detritus or plankton feeders using ciliated tentacles to intercept food, others may lead a free-moving life and are usually predatory and have an eversible pharynx with strong jaws and teeth, see Neris virens below left. Usually they have a well-defined head and no clitellum (see Oligochaetes).

Arenicola sp. (left and below left) are often dug up for bait by fishermen who find them by searching for worm casts on the surface. The worm comes to the surface and defecates every 40 minutes or so, then returns to its mucous-lined borrow. In the image above left you can see the worm's head is located towards the "blind" end of its burrow. It continually eats the sand in front of it, and this action forms a small depression on the surface. When the tide is in small prey items and detritus fall into this depression and steadily move towards the worm's mouth. Any organic matter between or on the sand grains is digested and the waste grains secreted on to the surface.

Neanthes virens, above and left is also known as the king ragworm, sandworm, and clamworm. Is omnivorous and hunts for food in the sand and can grow up to 90 cm long. When fully grown it will have more than 200 segments. It is found in the North Atlantic and is sometimes harvested for fish bait.

Sabella sp. below left are commonly known as fan worms, secretes a parchment-like tube made of sand grains mixed with mucus. Cilia on the radioles direct small food particles down the grooved radiole to the mouth. Sand grains are also directed towards the mouth. Grains of a suitable size are passed into storage sacs until they are needed for tube building. The walls of the storage sacs produce the mucus which is mixed with the grains of sand used in building the tube. This mixture is extruded in a slimy rope to add to the top of the tube.

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Pomatoceros triqueter, the keel worm above, whose calcareous tubes are on the right is found in the NE Atlantic and the Mediterranean on hard surfaces. The tubes are up to 1 cm in diameter and 12 cm long. There is usually a visible keel running along the tube - which gives the worm its common name. The worm emerges into the open and feeds with its tentacles which are often red.