Abdomen: Posterior part of the body. In Ampharetidae and Melinnidae it does not bear notochaeta.

Achaetus: Without chaeta.

Acicular: Needle-shaped

Anterior acicular neurochaeta: In Melinnidae, neurochaeta starts before the notochaeta. These anterior chaetae are acicular while more posterior neurochaeta are uncinigerous.

Branchia: A respiratory organ. In Ampharetidae and Melinnidae, there can be two, three or four pairs of branchiae. The branchia originate from several segments but appear to be clumped on segment II and III, often they will be fused forming a branchial ridge across segment II. In Ampharetidae they can easily break off, leaving scars of attachment. In Melinnidae they are unlikely to fall off.

Chaeta: Chitinous structure protruding from a parapodium.

Chaetiger: A segment with chaeta.

Cilia: Slender, microscopic, hair-like structures.

Cirriform: Slender and cylindrical.

Cirrus: A slender and cylindrical appendage.

Dorsal crest: A membranous fold forming an anteriorly open pocket behind the branchiae in Melinnidae.

Elytra: Modified dorsal cirri typical of scale worms or Aphroditiformia. They are shield-like scales that are attached dorsally on notopodia and that generally cover the dorsum of individuals. There is maximum one pair per segment. Their number, size, arrangement, and ornamentation are important taxonomic characters.

Epitoky: A process that occurs in many species of marine annelids where in a sexually immature benthic worm, called the atoke, goes through some morphological transformations into a sexually mature pelagic form specialized for swimming called the epitoke. Unlike atokes, epitokes can reproduce sexually. 

Eversible pharynx: Several annelids are capable of turning the pharynx inside out for feeding purposes. This is called an eversible pharynx. The pharynx may be smooth or armed with protuberances. Protuberances may be wart-like, soft papillae, or hard sclerotised paragnaths (as in Nereididae). The pharynx may be delineated in different parts, in Nereididae designated parts are called Area's which are important in descriptions.

Eyespot: A simple, light-sensitive organ appearing as a pigmented spot. On the prostomium they can occur in pairs or patches. Can also be found on the pygidium. The eyespots usually fade away in alcohol.

Facial tubercle: Scale worms, for example Polynoidae and Sigalionidae, may have a protuberance on the anterior margin of the head in addition to the median antenna.

Fanworms: Annelids in the families Fabriciidae, Sabellidae and Serpulidae.

Filiform: Threadlike.

Head: The head of ampharetids and melinnids consist of fused prostomium, peristomium and one achaetus segment (segment I). The borders between these parts are difficult to distinguish.

Ligules: Lappets in the parapodium in species of the family Nereididae are called ligules, and divided in dorsal and ventral ligules in technical descriptions.

Limbate chaetae: Simple capillary chaetae with a subdistal enlargement, a limbus. These chaetae are common in fanworms – Fabriciidae, Sabellidae and Serpulidae.

Lower lip: A muscular structure ventral to the mouth. In some species enlarged and extended anteriorly and laterally.

Modified notopodia: A pair of notopodia wich are elevated and/or modified compared to neighbouring notopodia. 

Neuropodial dorsal cirrus: A cirrus above a neuropodium.

Neuropodium: The ventral part of a parapodium. In most ampharetids and melinnids, the shape of thoracic and abdominal neuropodia differ. In the thoracic type neuropodia, the uncini are located in a furrow. In the abdominal type neuropodia, the uncini are located at the margin of the neuropodia. The change from thoracic type to abdominal type changes abruptly without any transitional segments. However, the change does not necessarily correspond to the change from thorax to abdomen.

Neurochaeta: The chaeta on a neuropodium.

Notopodium: The dorsal part of a parapodium.

Notochaeta: The chaeta on a notopodium.

Nuchal organs: Paired chemosensory structures situated postero-laterally on the prostomium, can be seen as pits or grooves.

Operculum: Typical of members of Serpulidae. Modified radiole, generally without pinnules, that is used to plug the tube when the worm is retracted. Opercula may have distal calcified of chitinized structures that are often ornamented and have taxonomic value for species identification.

Paleae: The notochaeta on segment II in ampharetids. Paleae are absent in melinnids. The paleae can be stout and large, but also small and unnoticeable and even missing.  The number of paleal chaeta can also be a helpful character when identifying ampharetids, but care should be taken since the number is size-dependent (juvenile/adult). Some authors do not separate paleae from other notochaeta when counting the number of thoracic chaetigers.

Papilla: A relatively short (shorter than a cirrus), rounded appendage.

Parapodium: A fleshy lateral projection from a body segment, usually bearing chaeta. In ampharetids and melinnids, the parapodia are biramous, divided into a notopodium and a neuropodium.

Peristomium: Presegmental region of the body that lies between the prostomium and the first segment, surrounding the mouth. The peristomium forms a ring behind the prostomium and extends forward as a lower lip.

Postbranchial depression: The area between the branchia, the dorsal crest and lateral ridges with the anterior chaeta in Melinnidae.

Postbranchial hooks: A pair of stout hooks located behind the branchiae in Melinnidae.

Prostomium: The anteriormost, presegmental part of the body.

Prostomium trilobed: The prostomium is divided into a middle lobe and a posterior lobe surrounding the middle one on three sides, so that the anterior prostomial margin is trilobed.

Pseudoperculum: Typical of members of Serpulidae. Modified radiole, generally without pinnules; can develop into a new operculum when this one is lost.

Pygidium: The posterior most part of the body bearing the anus. It is achaetus, but it can have appendages.

Rudimentary notopodia: The term used to describe various more or less prominent dorsolateral protrusions on abdominal segments. These protrusions are different types of structures and, in some cases, not notopodial in origin. Often, they are ciliated, and depending on the length of the cilia, or if the cilia have fallen off, the protrutions can be more or less obvious.

Segment: One of the serially repeated units comprising the body. Usually written with roman numbers. The prostomium, peristomum and pygidium are not considered to be segmental.

Tentacles: Feeding appendages. They are attached inside and can be retracted into the mouth.

Thorax: The anterior part of the body. In Ampharetidae and Melinnidae, it is the part of the body which bears notochaeta.

Unciniger: A segment with uncini.

Uncinigerous: With uncini.

Uncini: Short beak-like chaeta with many teeth