The phylum Annelida is composed of segmented, vermiform ("worm-shaped") organisms represented in nearly all terrestrial and aquatic environments. This major phylum contains more than 22,000 species distributed among two currently recognized classes, the Polychaeta and Clitellata. Annelids possess coelomate bodies divided into segments, as well as chitinous spines or chaetae (lacking in leeches). Segmentation (metamerism) appears to have evolved separately in annelids and arthropods; annelids are more closely related to mollusks and other invertebrates that produce a trochophore larvae. Annelids are represented in the fossil record dating back to the Ediacaran Period (580 million years ago), and the clitellates appear to have evolved from the polychaetes. The phylum Annelida may also contain the Echiura, a group of nonseg-mented worms, and the Myzostomida, a group of echinoderm parasites.
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