Glycera mimica - a bloodworm
Polychaetes, or bristle worms, belong to the Phylum Annelida. This phylum is characterised by a segmented body, possession of a true coelom (the body cavity between the central gut and the body wall), a pre-segmental section anterior of the mouth called the prostomium, a nervous system comprising of a brain and a pair of nerve cords running the length of the body with a pair of ganglia (concentrations of nerve cells) in each segment. Other members of this phylum are the Oligochaeta (earthworms and their relatives) and Hirudinea (leeches).
A loose definition of members of this class is: marine aclitellate annelids usually bearing numerous chaetae emanating from parapodia (outgrowths on each segment). They are a diverse and specious group found mostly in the marine environment (although there are freshwater and terrestrial species). Within soft sediment environments, they are among the most dominant and species rich taxa.
Pygospio elegans - a spionid worm
This makes them ecologically important and ideal organisms for environmental assessment because, in addition to their abundance and diversity, they are generally quite small and relatively sessile giving clues to the biotic response to environmental change.
There are about 80 families of polychaete organised into about 17 orders. Several researchers are currently working on the phylogenetic relationship of the Class and their results should be out soon.
If you would like to know more then have a look at the annelid discussion group.
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