any worms of the subphylum Annelides, which consists of the most highly developed coelomate worms. Some zoologists classify the annelids as an independent phylum (Annelida). The body, ranging from several mm to 3 m (tropical earthworm), is bilaterally symmetrical and is divided internally by septa into segments, or somites. External rings, or annulations, correspond in number to the somites. The name of the worm is derived from the annulations. There may be as many as several hundred segments. Metamerism of the internal organs results from the body segmentation. On each segment there may be lateral appendages with setae; these are rudimental extremities, or parapods. The musculocutaneous sac consists of a thin cuticle, the peritoneal epithelium, and longitudinal and circular muscles.

The circulatory system is closed, with two main vessels, the dorsal and the ventral, which are connected by annular vessels. Respiration is cutaneous, and sometimes there are gill filaments. The excretory organs are metamerically paired nephridia. The central nervous system is composed of the brain, the subpharyn-geal ganglia, and the ventral nerve cord. Annelids are dioecious or hermaphroditic. Cleavage of the egg cells is spiral. The development of annelids is direct or larval (trochophore stage). In addition to sexual reproduction, there is asexual budding, or paratomy.

Annelids are divided into four classes. The most diverse class, Polychaeta, comprises over 6,000 species and is predominantly marine. The class Oligochaeta has approximately 3,000 species, which are found in the soil (earthworms) or in fresh water. There are approximately 300 species in the class Hirudinea (leeches). They are found in fresh water and in seas; some leeches live in damp soil among rotting leaves in tropical regions, and some species of Hirudinea are parasitic. The class Euchiuroidea consists of a few species of marine worms that burrow in the bottom of the sea. They lack body segmentation.

There are several theories of the origin of the annelid, the dominant one asserting it descended from the lower platyhelmin-thes, or flatworms. Polychaeta is considered to be the oldest class of annelids. As a result of the transfer to fresh water and terrestrial living, the oligochaetes descended from the polychaetes. The leeches, in turn, developed from the oligochaetes. Annelids have existed since the Middle Cambrian period.