The body of a polychaete consists of four zones: prostomium, peristomium, trunk and pygidium. The prostomium is the lobe anterior to the mouth. The peristomium is the lobe around the mouth. Pro- and peristomium together are often called ‘head’ but in some families the first segments are also part of the head. In fact, it is often hard to tell where the head ends and the trunk begins. This is due mainly to the peristomium. This can be so small as to be almost invisible, it can be fused to the prostomium and/or to the first segment or well developed or even biannulate. So one can never be sure whether the first ring behind the prostomium is the peristomium or one of the first segments, unless you are already familiar with the body plan of the worm you are studying. In order to overcome this difficulty, it is common practice to count chaetigers rather than segments. The anteriormost chaetae are usually easy to find.
The prostomium bears antennae (often one mid-dorsally and one or more pairs antero-laterally) which are essentially tactile and a pair of palps ventrally, preceding or flanking the mouth, which are chemosensory and used mainly in feeding. One or more pairs of eyes, often large and frequently having a lens, are mounted dorsally. Ciliated nuchal organs of various designs and functions may pass back dorsally from the rearward edge of the prostomium. The peristomium usually has appendages that may have originated in several ways but they can be referred to, generally as tentacles. These may be short or long, flexible (and perhaps contractile or retractable) or rigid although, quite often, they are similar to the antennae. Tentacles may be mobile and ciliated for food gathering; amongst the more specialised tubeworms, they are stiff and feathery, forming a funnel-like branchial crown used to generate a current for respiration as well as suspension feeding.
The anterior part of the gut very often can be everted through the mouth to form a proboscis. Its lining is frequently elaborated into fleshy or cuticular papillae, hard ridges, or chitinous teeth; in the more rapacious errant species there may be two or four dark, chitinous, opposable jaws opened and closed by strong muscles. The arrangement of teeth on the proboscis in some species, or the pattern of the elaborate pharyngeal jaws, is important in their identification.
Some burrowers use a relatively enormous proboscis as a kedge-anchor to help draw them through the sand.
All segments are, by definition, part of the trunk, i.e., the prostomium, peristomium and pygidium are no segments. The segments usually bear 'feet' or parapodia.
The parapodium can be divided into dorsal and ventral parts (rami) known as the notopodium (top part) and neuropodium (the nerve cord is ventral), respectively. The major part of each is usually a fleshy lobe containing a bundle of bristles (chaetae, often referred to as setae); hence, those in the dorsal bundle are called notochaetae and those in the ventral bundle, neurochaetae. The lobes are usually supported internally by strong chitinous rods, acicula, whose outer end sometimes protrudes like a short, stout bristle. Each lobe may be subdivided and more than one bundle of chaetae may be present; alternatively, one entire ramus (most often, the notopodium) may be lacking. Each ramus, if present, typically has a cirrus associated with it. They are usually tactile, resembling a small antenna, although the dorsal one may be modified into a paddle-blade, a branching gill or a protective scale. In burrow- or tube-dwelling worms, the fleshy lobes and other appendages may be much reduced.
In many species all parapodia are similar throughout the length of the body, but in others their shape may change gradually or abruptly. If there is an abrupt change in the arrangement of chaetae the anterior part of the trunk is called thorax, the posterior part abdomen.
Chaetae occur in a variety of forms. They may be simple (all in one piece) or jointed (also called compound chaetae), usually with a short terminal part (often hooked) hinged to a longer shaft. The simplest in form are fine, hair-like capillary chaetae; other simple chaetae may be stout, distally serrated, or hooked. Often a straight shaft has a swelling, decorated with coarse teeth, from which the blade-like end region tapers away at an angle. In species that swim well, especially the sexual epitokes of various errant families, they may be flattened to resemble oars. Burrowing species often have stout hooked chaetae, or crochets, by which they can anchor themselves to the walls of the burrow.
Tubeworms use, for this purpose, uncini which are small chitinous plates whose outer edge is serrated, mounted in a stacked row within the groove of a parapodium modified in the form of parallel swollen lips: the arrangement is known as a torus and resembles a short, closed zip-fastener. The usual layout is for notopodia to have a bundle of normal bristles and the associated neuropodia to have such a row of uncini or crochets. In the most specialised tubeworms, the arrangement in the 'abdomen' is the reverse of that in the 'thorax'.
Chaetae occur only infrequently on the peristomium and not on the pygidium which, however, often has a pair of anal cirri. In some groups, the anus is surrounded by papillae or lobes whose form and number may be important in identification.
Other terms are introduced in the glossary.