The main difference between annelids and all other types of worms is that annelids have segmented bodies. Annelids also have a coelom, which is basically another tube inside the body that is filled with fluid and contains many of an annelid's internal organs.
Annelids are considered lophotrochozoas and are actually more closely related to mollusks than any other type of worm, such as those found in the phylum Nematoda or Platyhelminthes. In fact, there is no official group known as worms, as this term is generally applied to any invertebrate that has a long, thin body.
There are more than 13,000 known species that belong to the phylum Annelida, which can range in size from almost microscopic to more than 3 meters in length. The longest species of annelid are the giant Gippsland earthworms that are found in Australia.
Like other types of worms, annelids generally do not have a distinguishable head or tail, with no antennae or other visible appendages. The bodies of all annelids consist of one tube contained inside another larger tube. Each segment of an annelid's body is nearly identical. The only exception is some of the end segments, which may be more specialized and are usually the only segments to contain sex organs.