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. Latin: anulus = ring
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.
(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 on paddle-like extensions from the body wall called parapodia (see the drawing of Nereis virens below). 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.
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 above right and Neanthes virens, below. 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).