Annelids belong to the phylum Annelida and they are the diverse group of organisms. This group contains species like earthworms, leeches, and marine worms.
Further the annelids can be classified in to three classes. They are:
The class Oligochaeta includes freshwater and terrestrial worms, like Lumbricus (earthworms). The members of Oligochaetes are monoecious. They have well developed metamerism and apparent posterior segments. The head is degenerated with the sensory segments. Oligochaeta also have a spine like appendages and they also have a reproductive structure called the clitellum.
The class Polychaetes is structurally different class of the annelids and is completely marine creatures. This class includes marine worms like Nereis and fresh water species. They are free moving organisms. Polychaeta have the most typical body structure and they have many fin like appendage called parapodia. in some species that belong to this class the fin like appendages cover the entire body that gives a fuzzy appearance and in some cases the fin like appendages re poisonous. The species that belongs to this class lack the clitellum and are mostly dioecious.
This class Hirudinea includes the freshwater species like leeches. They have an anterior and posterior sucker. They are monoecious and asynchronously (i.e. an individual can switch between from a male to female and vice versa). Most of the species that belong to this class are predacious.