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A new genus of ectoparasitic marine annelids living on ctenophores, Ctenophoricola gen. nov., is described and its feeding behaviour, reproduction and developmental stages are discussed. Its unusual morphology challenged its placement within the known marine families. However, analyses of mitochondrial and nuclear sequence data showed the new genus as member of the Alciopini, a group of holopelagic annelids included within the Phyllodocidae. Ctenophoricola masanorii sp. nov. from Japan and Ctenophoricola rousei sp. nov. from the Canary Islands (Spain) are described. A third species from the Gulf of California is not formally described because the specimens are in poor condition. The new genus is characterized by having: 1) two distinctive body regions, the anterior with reduced parapodia lacking chaetae, and the posterior with long parapodia and chaetae and 2) a pair of large, elongate lensed eyes. These eyes are here described using histology and 3D reconstruction based on a Californian specimen. The two new species mainly differ in colour pattern, shape of parapodia, number of chaetae and body ciliation.

Dorvilleidae is a morphologically heterogeneous group, including some of the smallest described annelids with adult length of only a few hundred µm and only a few segments (Neotenotrocha). Others may attain a size up to 10 cm and have many segments (e.g. Dorvillea). The colour of live animals is variable, transparent to whitish being common (Pl. 7b–d; Fig. 31.2a), but sometimes with bright red or other pigmentation (Pl. 7a). Within Dorvilleidae, Ophryotrocha is well known through a series of studies (Sella and Ramella, 1999 and references within) for an impressive variation in mating systems. Interestingly, taxa showing no apparent morphological differentiation are separated by their differing reproductive habits. Some Dorvilleidae (e.g. Ophryotrocha) are also easily kept in culture and so provide excellent subjects for experimental studies.

Amphinomida includes two major groups: Amphinomidae and Euphrosinidae. Amphinomidae occur mainly in littoral warm waters. Some are called fireworms and are known for their brittle chaetae that easily break when touched and produce a burning sensation.

Siboglinidae is among the most famous and well-studied clades of Annelida, though until recently they were treated as one or two phyla, Pogonophora and Vestimentifera, separated from Annelida.

Magelona (‘shovelhead worms’), contains over 70 described nominal species that are relatively uniform in appearance. They have a somewhat unusual morphology and have historically been difficult to relate with other annelids. They are now regarded as sister group to Oweniidae, and as Oweniida, are the sister group to all other living Annelida (see Chapter 1 regarding the inappropriateness of the unfortunate name Paleoannelida that has been proposed for this group).

Members of Sabellidae (feather-duster worms) are among the most easily recognizable of annelid groups due to their possession of an often spectacularly colourful radiolar crown and by the mucus/ parchment/ sediment tubes that they generally inhabit (Pl. 8a–d).

Lopadorrhynchidae is a small group of holopelagic annelids. A common misspelling of the name is Lopadorhynchidae with a single ‘r’ (Read and Fauchald, 2021).

Cossura live in some shallow marine sediments but are more common in mixed sand and mud sediments in deeper seas. Around 30 accepted species have been described to date. They are recognizable by the thin mid-dorsal branchia that can be nearly as long as the body originating from an anterior segment.

Iphionidae are highly characteristic scale-worms with an elliptical outline, somewhat like chitons (Figs. 9.1a, b). The adult size is usually 1–3 cm for up to 39 segments.

Lumbrineridae adults are medium sized to large annelids, ranging from a few centimetres to over half a metre in length and several hundred segments (Pl. 6a).

Fabriciidae were for many years considered to be part of Sabellidae and share similarities such as the radiolar crown and dwelling in a mucus/sediment tube (Figs. 40.2a, b)

Phyllodocidae are sometimes known as paddle-worms owing to their (usually) large and flattened dorsal cirri. The holopelagic Alciopidae (see Pl. 5h) are previously treated as a separate taxon in the literature but is here included in Phyllodocidae as Alciopini.

Despite its minute size, the first member of Nerillidae (Nerilla antennata) was described in 1848 by Schmidt. In his original description he admits initially mistaking Nerilla for the juvenile of a known worm and becoming aware of his discovery only after finding mature specimens.

Glyceroidea include two clades Glyceridae Grube, 1850 (often called‘bloodworms’) and Goniadidae Kinberg, 1866.

Acoetidae are large annelids with many segments and are unusual among scale-worms in being tubiculous (Fig. 8.1a).

Hesionidae is named after the mythical Hesione, princess of Troy, who was rescued from a sea monster by Hercules. This group of beautiful worms are not uncommon in the sublittoral, especially on rocky and mixed bottoms, and they also seem to thrive in deep-sea environments with many species described from hydrothermal vents (Fig. 15.2c), whalefalls, and methane seeps.

Hartmaniellidae is monotypic for Hartmaniella with the former being superfluous, we here just use the latter. Hartmaniella is known from a few records between depths of 45 and 210 m.

Nereididae (littoral ones often referred to as ‘ragworms’) are probably the most familiar of annelids, both from textbooks in zoology and marine biology and from their occurrence on seashores around the world. They are widely employed in teaching, in laboratory experiments, and as bait for fishing.

Compared with most annelid groups, Cirratulidae has been well studied in terms of biology, physiology, and population structure, but the overall systematics and taxonomy of the group remains quite problematic.

Protodrilida (‘first’ or ‘original’ ‘worms’) is a clade that currently comprises around 65 accepted species that are known historically for being members of the now defunct taxon Archiannelida.

Lacydonia is a small and morphologically very homogenous group in Phyllodocida about which we know very little. The adults have a relatively small number of segments, and the reported length is up to 8 mm; the body shape in dorsal view is cylindrical with tapering anterior and posterior ends (Pl. 5c).

Serpulidae (fanworms), like Sabellidae, are very well-known owing to their colourful radiolar crown (Pl. 8e–g). Serpulidae are usually easily distinguished from Fabriciidae and Sabellidae in that they live in a calcareous tube, though this is sometimes embedded in coral.

Microphthalmidae is a small group within Phyllodocida. Herein, we follow the delineation of the group by Salazar-Vallejo (2019a) and include Hesionides as well as Struwela. Both Microphthalmus and Hesionides are interstitial animals.

Oweniidae is a taxon with small membership that, until recently, was more famous for its larval form, the unusual and beautiful ‘mitraria’ (Fig. 77.1g), than for the adults.

Sabellariidae (known as honeycomb worms or sandcastle worms) build distinctive hard tubes from glued sand or other coarse particles and are commonly seen forming colonies on rocky surfaces in intertidal marine environments (Pl. 9a).

Myzostomida (from the Latin myzo to suck and stomum for mouth) is an unusual and in some ways diverse group of worms(Figs. 35.1, 35.2, 35.3, 35.5), comprised of more than 180 nominal species (Read and Fauchald, 2021. Lacking a common name, they are generally referred to as ‘myzostomes’.

Paraonidae (no common name, but jesting students often call them ‘paranoids’) is a relatively small group in taxonomic terms and, because they tend to be found in deeper waters, there is little known about them.

Annelids (the segmented worms) exist in a remarkably diverse range of mostly marine but also freshwater and terrestrial habitats, varying greatly in size and form. This text provides. This text begins with an introduction to the phylum and an outline of annelid taxonomy. The book describes their collection and the methods to ensure their optimal preservation, and provides an overview of anatomy with its relevant terminology. It includes the latest molecular phylogenomic evidence and is organised based on a new, robust phylogenetic hypothesis. It looks at groups which include Clitellata (comprising more than a third of total annelid diversity), Sipuncula, and Thalassematidae (formerly Echiura). It reflects the enormous amount of research on these organisms that has burgeoned since the millennium, principally due to their use as model organisms to address wider and more general evolutionary and ecological questions.

Typhloscolecidae are all holoplanktonic and have a cylindrical body. Adult animals range from 0.5 to 4 cm in length with up to 50 segments, although many taxa have a segment number not exceeding 20–25. This suggests a fixed adult number of segments in some taxa.

Syllidae constitutes a large and variable, but nevertheless highly characteristic group of polychaetes. The name comes from Syllis, a nymph in Greek mythology, who was the lover of the god Apollo and the mother of king Zeuxippos of Sikyon.

Named for their strange anatomy, Aberranta is a small, rare, and morphologically homogenous and characteristic group of free-living annelids, with adult length up to near 7 mm for about 40 chaetigers.

Quillworms (Hyalinoecia) and the Australian beachworms (Australonuphis) are among the better known Onuphidae; the former with highly characteristic tubes (Pl. 6f) and the latter for their size (up to several metres) and occurrence on sandy beaches and their importance as bait in fisheries. Overall, Onuphidae vary in size from a few millimetres to several metres, although sizes around a few centimetres are common. The anteriormost part is often cylindrical and widest, becoming medially and posteriorly more flattened. Many of these animals are tubicolous. Some (e.g. Australonuphis) have a thin and ephemeral tube. In others the tube is tough and may be highly characteristic, such as in Hyalinoecia, which construct a cylindrical, translucent, and quill-like tube (Pl. 6f), or in Nothria conchylega where it is distinctly flattened with an inner lining of parchment and large pieces of shells on the outside (Pl. 6h). Live specimens are iridescent and may have characteristic red or brown pigmentation patterns, often as transverse segmental bands; others are unpigmented (Pl. 6f–h).

Pontodora is a small and poorly known holopelagic group including only the two described taxa Pontodora pelagica Greeff, 1879 and Epitoka pelagica Treadwell, 1943.

Opheliidae are commonly found in sandy or muddy substrates from intertidal regions down to more than 7000 m. They are often noticed when disturbed by rapidly swimming in an energetic way and some

Ampharetidae are tubicolous, as are most members of Terebelliformia, and share with Alvinellidae and Melinnidae the feature that their multiple grooved palps, usually called tentacles, can be retracted into the mouth. Typically, Ampharetidae are 10 to 60 mm long, though some adults are only a few millimetres long.

Clitellata is clade of hermaphroditic, direct-developing Annelida found worldwide in most aquatic and terrestrial habitats. They comprise nearly half of the total known species diversity of Annelida, with an estimated 8800 accepted species.

Nephtyidae have a well-deserved difficult reputation among workers involved in benthic monitoring who try to identify them. They can be very abundant in shallow water sediments and there are often several different nephtyids in the same sample.

The multiple grooved palps (‘tentacles’) of Terebellidae gives rise to their common name, spaghetti worms. These tentacles are a common sight extending out over the substrate in shallow marine waters worldwide. When disturbed the tentacles, often brightly coloured, can be seen coiling and looping as they are retracted back towards the worm.

For many years this group was referred to as Echiura, with the rank of phylum following Newby’s (1940) embryological study. The group is now consistently recognized as the sister group to

Paralacydonia is a small and morphologically homogenous group of Phyllodocida. As may be guessed from the name they were thought by Fauvel to be close to Lacydonia, yet their position within Phyllodocida still remains unresolved today.

Orthonectida, from the Greek for ‘straight’ and ‘swimming’, is a small group of marine Annelida that are parasites of Acoela, other Annelida, Echinodermata, Mollusca, Nemertea, Platyhelminthes, and Xenoturbella (Kozloff, 1992; Nakano and Miyazawa, 2019).

Dinophilus was erected by Schmidt (1848) and the name refers to the whirling motion these worms can show owing to their ciliary movement. Dinophilidae are all small, at less than 3 mm long as adults, and lack chaetae and obvious segmentation.

Flabelligeridae, with a common name of bristle-cage worms, are generally recognizable by their papillose body, often coated with thick covering of sediment and the fact that the head is retractable between anterior segments which may project forward a ‘cage’ of chaetae (Pl. 14b–d).

Polynoidae is the largest group of Aphroditiformia, and one of the more species-rich of annelid clades covered in a chapter here. The body is dorso-ventrally flattened and often has an oval shape, but some can be long and slender (e.g. Polynoe scolopendrina; Pl. 2e).

Alvinellidae (‘Pompeii worms’ or ‘palm worms’), are a relatively recently discovered group known only from sites associated with deep-sea hydrothermal vents in the Pacific and Indian Oceans.

The annelid body consists of three fundamental regions. There is always a presegmental region derived from the larval episphere, anterior to the prototroch region around the mouth.