The largest class of the phylum Annelida, containing 68–70 families. About 1600 genera and 10,000 species have been named from worldwide areas; about one-fourth of this number may be synonymous. Polychaeta (meaning “many setae”) is conveniently though not clearly divisible into the Errantia, or free-moving annelids, and Sedentaria, or tubicolous families.
The body may be long, cylindrical, and multisegmented, or short and compact, with a limited number of segments. It consists of prostomium (Fig. 1), or head; peristomium, or first segment around the mouth; trunk, or body proper; and tail region, or pygidium. Most segments have highly diagnostic paired, lateral fleshy appendages called parapodia. These are provided with secreted supporting rods and spreading fascicles of setae, or hooks, which display remarkable specificity.
The anterior end, or prostomium, may be a simple lobe derived from the larval trochophore, modified as a pseudoannulated cone, or covered by peristomial structures so as to be invisible. Oral tentacles for food gathering may be eversible from the buccal cavity; they may be long, slender, or thick and their surface smooth or papillated.
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The anterior, preoral end may be developed as a thick, fleshy papillated, nonretractile proboscis (Fig. 2), or the prostomium may be completely retractile into the first several segments and protected by a cage formed of setae directed forward, or concealed by a compact operculum formed of setae of the first several segments. The anterior end of the alimentary tract is muscular or epithelial; it may be covered with soft papillae or hard structures. These structures function for secretion, food gathering, and maintaining traction; they are named for their form or function.
The trunk is the main body region and is composed of metameres numbering few to many. They may be similar to one another (homonomous) as in Errantia, or different (heteronomous) resulting in anterior thoracic and posterior abdominal regions.
Reproduction is highly evolved and diversified; it can be sexual or asexual. Sexual reproduction is usually dioecious, with the two sexes similar. In rare cases it is dimorphic.
Polychaetes range in length from a fraction of 0.04 in. (1 mm) to more than 144 in. (360 cm). Colors and patterns are varied and specific, due to pigment and refraction of light. Littoral, warm-water species may be brilliant and multicolored, whereas polar and deep-water species tend to be drab or sometimes melanistic to almost black.
Most polychaetes are free-living; some of the remaining members are commensal with another animal for attachment surface, for food, or for protection. Polychaetes are distributed in all marine habitats and show remarkable specificity according to latitude, depth, and kinds of substrata. Most of the families tend to be represented in any major geographic area, although taxa may differ with place. See Annelida
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