Because of this finding, the Oligochaeta and the Hirudinea are now referred to as the Clitellata, an old taxon that dates back to 1919. The closest relative of Hirudinea (Hirudinoidea) is actually found inside the Oligochaeta and is the family Lumbriculidae. Instead, the terms “oligochaete” and “polychaete” are simply used as informal, common names. The Oligochaeta and Polychaeta are currently determined to be paraphyletic groups and no longer hold any systematic ranking. However, more recently, all three groups are no longer recognized.
The phylum was previously divided into three classes as follows: Polychaeta (marine, freshwater, and terrestrial polychaetes, about 12,000 species), Oligochaeta (earthworms, about 10,000 species), and Hirudinea (leeches, about 700 species). Phylogenomic and other molecular phylogenetic analyses have shown that taxonomic groupings previously based on morphology in many cases are invalid. The overall classification of the phylum is undergoing significant revisions and has not yet been stabilized completely. Several species of annelids can be found in Arkansas. The term “Annelida” originated in 1802 from French naturalist, Jean-Baptiste Lamarck’s (1744‒1829), annélides. Annelids are bilaterally symmetrical, triploblastic invertebrates that inhabit a wide diversity of habitats, including marine environments such as tidal zones, hydrothermal vents, lotic and lentic freshwater habitats, and moist terrestrial habitats. They include earthworms, leeches, and ragworms. The phylum Annelida consists of over 22,000 living species of segmented worms.
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