The three classes of organisms in the phylum annelida include freshwater worms and earthworms (oligochaetes), marine worms (polychaetes) and leeches (hirudineans). Together, these three classes contain 12,000 known species that live in various habitats, ranging from oceans to mountain glaciers.
One distinguishing characteristic of members in the annelida phylum is the segmented body. Each segment is known as a metamere and contains organs responsible for different functions that take place within the body. The anatomy of the annelid body consists of a head, the trunk containing metameres and a terminal region called a pygidium. The pygidium region is not segmented.