English: Annelids; Ringed worms; from Latin anellus, little ring.
Zoology: 17000 species; segmented worms; including ragworms, earthworms and leeches; marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments; bilaterally symmetrical, triploblastic, coelomate; parapodia for locomotion.
Taxonomy: the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species, recent research has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition the Pogonophora,
Echiura and
Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the
Lophotrochozoa, a "super-phylum" of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans.
The basic annelid form consists of multiple segments each of which has the same sets of organs and is divided from its neighbors by a septum. However septa are poorly-defined or absent in some species, and
Echiura and
Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles to act as hearts. The septa of these species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis ("ripples" that pass long the body) or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniquies, some burrowing species evert their pharynges to drag themselves through the sediment. The species traditionally regarded as "polychaetes" generally have parapodia, which are often paddle-like and used for crawling or swimming.
Classes and subclassesClass
Polychaeta (paraphyletic?)
Class
Clitellata (see below)
Oligochaeta - earthworms, etc.
Branchiobdellida Hirudinea - leeches
Class Myzostomida
Class Archiannelida (polyphyletic)