The Annelida or segmented worms, form a major animal group, typically of elongated cylindrical shape and mostly ranging in length from 10 mm to 150 mm. The body is characteristically made up of a series of ring-like segments, indicated externally by annular constrictions. The relatively simple gut normally passes down the body from an anterior subterminal mouth to a posterior terminal anus. The first segment (prostomium) is situated above and in front of the mouth, the second (buccal segment or peristomium) surrounds it and the last (pygidium) encircles the anus. The segments in between primitively more or less resemble each other, each containing a similar complement of organs.
Annelida in the North Sea plankton are members of the Class Polychaeta.
[After Hayward and Ryland, 1990]