About 15,000 species of annelids are grouped into three
classes: the Polychaeta, mostly marine and a few soil and freshwater
bristle worms; the Oligochaeta, terrestrial and freshwater
bristle worms; and the Hirudinea, or leeches. There are about
9000 species of polychaetes, including myzostomarians, a group
of about 100 species of small polychaetes that live on or in echinoderms
(A-34), 6000 species of oligochaetes, and 500 species
of hirudinids.
About 15,000 species of annelids are grouped into three
classes: the Polychaeta, mostly marine and a few soil and freshwater
bristle worms; the Oligochaeta, terrestrial and freshwater
bristle worms; and the Hirudinea, or leeches. There are about
9000 species of polychaetes, including myzostomarians, a group
of about 100 species of small polychaetes that live on or in echinoderms
(A-34), 6000 species of oligochaetes, and 500 species
of hirudinids.
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