Annelids are commonly known as the Segmented Worms because of their distinct metamerism. They have undergone a great deal of cladogenesis, and their phylogeny is still being investigated and constantly updated.
metamerism reflected in external and internal anatomy
coelom serving as large, fluid-filled hydrostatic skeleton
A hydrostatic skeleton consists of closed or partly closed, fluid-filled chambers with flexible, muscular walls. Internal pressures generated by muscle contractions allow movement, with the muscles working against the fluid pressure as vertebrate muscles work against bone. The fluid-filled chambers also maintain the shape of the animal. Common examples of animals with a hydrostatic skeleton are earthworms, roundworms, etc.
well-developed nervous system with a cephalic ganglion & ventral,
ganglionated nerve cord
appendages present on each body segment (even if simply bristles)
metanephridia or protonephridia serve as excretory system
larva a characteristic form called a trochophore.
Annelida has undergone perhaps more drastic systematic revision than any other major phylum. Once considered to consist only of three major groups--Polychaeta (Marine Segmented Worms), Oligochaeta (Earthworms) and Hirudinea (Leeches)--the taxon has since been divided, regrouped, and re-divided many times. Molecular data have provided a clearer picture of Annelid phylogeny, at least until we get a new picture.
(Read the paragraph on this site regarding Annelid phylogenetic reorganization to get an idea of just how work-intensive this can be! And note that Dr. Krempels's professor for Invertebrate Zoology at U.S.C. was Dr. Kristian Fauchald, now of the Smithsonian Institute, who is one of the authors responsible for much of the currently accepted phylogeny.)