Image courtesy of The University of Arizona
The phylum Annelida is made up of segmented worms, numbering about 15,000 species. Body segmentation, a hallmark of annelids, was a major step in the evolution of animals. Annelids are protosomes, meaning they have a coelom made from cell masses. This coelom is divided into a series of repeated parts. This repetition is called metamerism, and each segment is called a metamere. There are a cluster of nerve cells and excretory organs in each metamere, but the ventral nerve cords, a dorsal and ventral blood vessel, and the digestive tract pass through the walls of segmentation and are therefore unsegmented. These walls, or septum, are thin sheets of mesodermic tissue, isolating the coelom. Except for the head and tail region, each with an opening of the digestive tract, making it a complete tract, each segment in an annelid is ringlike and very similar.
Segmentation allows for flexibility and mobility because annelids can bend at segmented parts. Therefore, because a segmented body is advantageous, it evolved twice, with the protosomes, as shown by phyla Annelida and Arthropoda, and again in the deuterosomes, as shown by phylum Chordata. Other hallmarks of the annelids are soft bodies that are round in cross section, repetition of organs in the segemented parts, and a body that is much longer than it is wide. There are three main classes in the phylum Annelida.
Below is a list of the phylum Annelida, with each class and the orders of each class (some of these classes, making up a very minor portion of the phylum, have not been described):
..Class Polychaeta -Polychaetes
Information provided by: http://www.sidwell.edu