|A mouth, muscular gut and anus|
|A body divided into segments; this may or may not be visible externally|
|The body cavity is a series of schizocoels often divided by transverse septa|
|The outer epithelium covered by cuticle with chaetae in most|
|Outer circular and inner longitudinal muscles|
|A closed blood system|
|Respiratory gas exchange through skin, gills, parapodia.|
|Sensory systems - touch, taste eyes (in some), photoreceptor cells, statocysts (in some).|
The Annelids are the segmented worms; there are about 15,000 species worldwide. In the U. K. there are 139 freshwater Annelids.
The body is divided into segments marked externally by circular grooves or annuli or septa. Latin: annulus = 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.
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, which are mainly marine and contains the rag worms, tube worms and fan worms among others, Oligochaeta, which are mainly terrestrial and freshwater, and contains the earthworms, and Hirudinea, the leeches. It is thought that the Hirudinea diverged from the Oligochaeta.Related pages