Annelida (worms and leeches)

Annelida overview

The Annelids are the segmented worms; there are about 15 000 species worldwide.

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 youngest 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 (below), 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.

Arenicola sp. (left and below) 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 burrow.

In the image 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.