Annelida: Some Systematics

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My goal here is not to go through all of the annelids taxonomically or to name all the families (such a task isn’t yet possible to do accurately, as annelid systematics is a mess and always has been, as this very straightforward 1967 quote by Day testifies: “The phylum Annelida to which the Polychaeta and several other groups belong, is difficult to classify into classes and orders.”). I will just highlight some taxa that I find interesting for some strange reason – not because they’re especially important.

As a historical note, there was briefly a third annelidan group besides Poly- and Oligochaeta called the Archiannelida, in which many basal annelids were placed. It’s now abandoned.

The polychaetes are the basalmost annelids and are most likely paraphyletic. Nonetheless, I’m grouping them together here for the sake of clarity. One characteristic is the reduction of the circular muscles, sometimes to the point of absence.


I want to write a small note on the nereidids because they have unique structures along their pharynx called paragnaths (the jaws at the end of the pharynx don’t count as paragnaths). They are hard and come in many forms and patterns and are commonly used to differentiate nereidid species. Keep in mind that unlike the jaws, the functional importance of paragnaths is unknown. The oral paragnaths (nearest to the jaws) have a role in keeping a tight grasp on food (the pharynx is rolled out then pulled back in when it catches food – the oral part is the only paragnath-containing part that comes in contact with food). They could also be used while burrowing.

The hard paragnaths and the jaws are assumed to be chemically similar (although paragnath chemistry has not been studied, afaik). The jaws are not made of chitin, as older invertebrate zoology textbooks say, but are instead made of zinc-rich proteins. Keep in mind that some species have only soft papillae instead of hard paragnaths (some have both together); they are likely to be the primitive state of the paragnaths or they could be homologous to them.

Nereidids are one of the groups that lost the ability to regenerate.


These are the honeycomb worms, so called because they aggregate in large, reef-like colonies. In fact, a larva will only settle if other sabellariids are around. The adults are immobile filter feeders and thus need high levels of oxygen and nutrient-rich water. Additionally, they need water with many suspended particles (shell fragments, sediment) which they use to build a protective tube. Their body is divided into three regions: the branchial crown, thorax and abdomen. The first of those is unique to sabellarids, and can be seen in the diagram below. It contains a circle of pinnules (small hairs) used for feeding and breathing.

Of the sabelariids, the most interesting, for me, are the serpulids (only because I have some in my fossil collection, though). These are tubeworms that build a tube made of calcium carbonate embedded in a mucopolysaccharide matrix. They can be distinguished from the other sabellariids by the presence of an operculum, a structure they can use to plug the tube as a defence mechanism (although it must be said that some serpulids don’t have one either).

Note that there is quite a mess in this area of the polychaete tree, with Sabellariidae, Sabellidae and Serpulidae always being shoved around different taxonomic levels. I am simply lumping them all together here.


Nothing much to say here. Typical opportunistic annelids, they’re found in places such as whale falls. Otherwise they’re found in other areas with unusually high organic content and periodic anoxia, such as in fish farms, hydrothermal vents or near sewer discharge.


There is one supremely cool species to be mentioned here: Scolelepis laonicola. They exhibit extreme sexual dimorphism where the males are dwarf ectoparasites of the females. The male drills into the dorsal side of the female. The head has no jaws and the nervous system is reduced, resembling the larval nervous system rather than the adult one; there is no brain or any cerebral ganglion. The muscles of the male extend into and attach to the inside of the female – the septa of the relevant segments in the female form a chamber around the penetrating part of the male. The coelomic cavity (i.e. where the organs hang around) of the male is filled with sperm. The human equivalent of this is not even imaginable… A horny headless midget jumps on a woman, punches through her back and ejaculates straight into her ovaries? I’ll let you decide.


This family includes the remarkable Osedax bone-eating worms that inhabit whale falls and other carcasses on the ocean floor. The Siboglinidae in general are characterised by a lack of a digestive system, instead relying on endosymbionts to get their food. Osedax drill into the bones and form a branching complex of tubes (called roots) where the trophosomes are found (where the endosymbiotic bacteria of the Oceanospiralles group live). They didn’t necessarily originate with whales though – they may have coradiated with them, but it is also plausible, and in agreement with fossil and molecular evidence, that Osedax, or its ancestors, was feeding on plesiosaur or mosasaur bones. Osedax is also cool because of the extreme sexual dimorphism, with dwarf, paedomorphic males living in harems in a gelatinous tube next to the female’s trunk. Human equivalent: Being part of a nymphomaniac’s sex slave ring. When you’re 5 years old. Siboglinids in general are dominant in cold seeps and hydrothermal vents and have no gut, mouth or anus.

They were first described in 1914 and until the 1970s, their feeding mode was a complete mystery. Some hypotheses put forward include that they rolled up their tentacles around food particles and extracellularly digested them; the same, but with the epidermis; or just simple diffusion through the trunk and tentacles.

Also included in the Siboglinidae are the Vestimentifera, a group long thought to be its own phylum consisting of animals specialised for hydrothermal vents and cold seeps in the deep ocean. They build calcareous shells which show several specialisations to their habitat. For example, the walls of their tubes are layered, and this helps the growth of aragonite (a form of CaCO3), overall reinforcing the tube.


These guys are important parts of coral reef communities. They either live in crevices formed by the reefs or simply burrow into the coral skeleton. Fishermen know them because many eunicids are used as bait; native tribes from various South Pacific islands eat the reproductive swimmers of the eunicid genus Palola. It’s reported to be a real delicacy.


The clitellates comprise ~5000 species, which can have anything from 7 to 600 segments, and is made up of the “oligochaetes” (paraphyletic) and the leeches. They’re characterised by their hermaphroditism; clitellates have a characteristic clitellum (you know, the thicker band of segments on an earthworm’s body), which is unique to the animal kingdom and produces a coccoon. Phylogenetically, there are other characters that ensure their monophyly. The most familiar one is the absence of parapodia: they are all legless (cf. leech and earthworm). They also have no nuchal organs, and their longitudinal muscle (annelids have longitudinal and circular muscle, allowing contraction) is one continuous sheet along the body, not split in each segment. Note that in clitellates, many phylogenetic characters come from sperm morphology: for example, clitellates are unique in having sperm with a single centriole, an acrosome tube and mitochondria aligned between the nucleus and the flagellum.

Just out of interest, the above picture shows an oligochaete phylogeny. It may be outdated, I’m not sure; but now at least you know how the sperms look like.

Just to make it clear, the leeches are not the sister group to the Clitellata; they are deeply-nested within them (i.e. they are highly-derived clitellates), forming a clade with Acanthobdella peledina (a parasite on Arctic salmon) and Branchiobdellida (freshwater crayfish commensals and parasites); this clade is in turn sister to the Lumbriculida.

It is unknown whether the ancestral clitellate was a terrestrial or freshwater organism. Traditionally, the oligochaetes have been considered to have an aquatic origin, either completely freshwater or brackish lagoons. As early as 1997, this has been challenged and a terrestrial alternative, based thoroughly on functional morphological explanations, was proposed and quickly gained popularity. However, the latter hypothesis, while apparently supported by morphology (e.g. clitellates have no nuchal organs, neither do any terrestrial polychaetes :o ), does not stand up to phylogenetic scrutiny, and an aquatic origin is more likely when the clitellates are studied from a historical evolutionary perspective (whether marine, freshwater or brackish is another story, that still needs resolution).


Fancy name for leeches. They are monophyletic, courtesy of their famous suckers, although the reduction of their coelum (and thus their hydrostatic skeleton) is also unique – this loss is probably related to their lifestyle, which does not require burrowing. They also have no sperm funnel, and their circulatory system is also reduced.

A known – yet rare in nature – member of the Hirudinea is the medicinal leech, Hirudo medicinalis. It is 10 to 15 cm long and lives in moors and other such habitats. Its back is dark green with six red-yellow or brown longitudinal bands, usually punctuated by black spots. In the Hirudinea, it belongs to the Gnathobdelliformes. It has three half-lens-shaped jaws in the mouth opening. They move synchronously and are surrounded by a ‘suction cup’ of sorts. The jaws rotate, cutting into the skin of vertebrates and are fixed on the skin by the suction cups. Their strong pharynx musculature allows them to suck up blood from their host, up to 10-15 ml.

H. medicinalis has glands producing hirudinin. This is used in helping against blood clots – so it is still used medicinally today. Other secretions from the leech have anti-venomous, anti-thrombic and immune-system-fortifying properties.


I only mention these guys because they’re one of the groups to lose the ability to regenerate. From their external morphology, there is only one character that defines their monophyly: the lack of copulatory organs. It’s in sperm that we have to look for the apomorphies, in this case a complex cytoskeletal network that connects the doublets to the cell membrane. Despite those two factoids, the naidines are actually the most species-rich clitellate clade (~800 spp.).


Fancy name for earthworms. They are also definitely monophyletic, with a large number of apomorphies, including a multilayered clitellum (the darker segments ~one fifth of the body) composed of more than 3 segments and lateral lines (similar to the lateral line in fish, they allow them to sense vibrations).

The most well-known member is probably the earthworm Lumbricus terrestris. No biologist has the right to call himself that if he has not dissected one of them. It’s the most common large earthworm in Central Europe. To move, it contracts its segments serially – the longitudinal muscles are relaxed and the circular muscles are used, and they contract each segment after each other (front to back), halving the body’s diameter in the process. This raises the pressure and causes fluids in the coelom to push forward. At each contraction, the prostomium is 2-3 cm ahead. The longitudinal muscles then contract,  the first segment and the circular muscles relax; the body is pushed to the ground. This happens in every segment serially. These contractions are controlled by each ganglion in the segments. This peristaltic process doesn’t help much when burrowing though; there they are aided by a scretion released from the mouth, loosening the substrate until they can swallow it – this is what is meant when people say that earthworms “eat soil”. As a sidenote, not all common earthworms are Lumbricus terrestris; an equally common genus is Allloboporus.

Jump to: Introduction, A Note on Spiralian Development, Annelids within the Metazoa
Picture Sources:

Jamieson, B. G. M. 1984. A phenetic and cladistic study of spermatozoal ultrastructure in the Oligochaeta (Annelida). Hydrobiologia 115, 3-13.

Orrhage, L. 1980. On the structure and homologues of the anterior end of the polychaete families Sabellidae and Serpulidae. Zoomorphology 96, 113-168.

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