<P> Since annelids are soft - bodied, their fossils are rare--mostly jaws and the mineralized tubes that some of the species secreted . Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years ago in the early Cambrian period . Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago . Palaeontologists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, and the earliest indisputable fossils of the group appear in the Tertiary period, which began 66 million years ago . </P> <P> There are over 22,000 living annelid species, ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus (Cognetti, 1922), which can both grow up to 3 metres (9.8 ft) long . Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids, most textbooks use the traditional classification into the following sub-groups: </P> <Ul> <Li> Polychaetes (about 12,000 species). As their name suggests, they have multiple chetae ("hairs") per segment . Polychaetes have parapodia that function as limbs, and nuchal organs that are thought to be chemosensors . Most are marine animals, although a few species live in fresh water and even fewer on land . </Li> </Ul> <Li> Polychaetes (about 12,000 species). As their name suggests, they have multiple chetae ("hairs") per segment . Polychaetes have parapodia that function as limbs, and nuchal organs that are thought to be chemosensors . Most are marine animals, although a few species live in fresh water and even fewer on land . </Li>

Annelida is the only phylum of the three with segmentation