Phylum: Annelida (Earthworms, Leeches, Polychaete Worms)

"Annelids are soft-bodied, segmented worms, comprising all worm-shaped organisms made up of many similar (sometimes not-so-similar), repeating units or segments. Each segment has two or more bundles of projecting bristles called chaetae. The annelids include the familiar earthworms, leeches, and many marine worms often called ragworms. Annelids are often common or abundant in moist soils and in marine and freshwater environments.

Other worm-shaped organisms that are not related to the annelids include nematodes (phylum Nematoda), ribbon worms (phylum Nemertea), flatworms and tapeworms (phylum Platyhelminthes) and wire worms (phylum Nematomorpha). None of these are segmented like annelids. However, a few small groups of unsegmented worms, previously classified in their own phyla, have recently been shown to be highly modified annelids. These include the peanut worms (previously phylum Sipuncula), spoon worms (previously phylum Echiura) and beard worms (former marine phyla Pogonophora and Vestimentifera). Although most annelids are clearly worm-like, they vary substantially in form. One enigmatic group, the Myzostomida, comprises about 150 marine species that live on or are parasitic on echinoderms (especially crinoids) that are scarcely worm-like at all.

Many annelids are ecologically very significant in their environments. Earthworms play a crucial role in soil formation, nutrient cycling and in maintaining plant productivity. In many marine and other aquatic environments oligochaete and polychaete annelids are often abundant and species-rich, again playing a key role in nutrient cycling and as food for fish and migratory shorebirds, among many other ecological benefits. Leeches were used (and misused) medicinally since mediaeval times and are finding modern uses in treating burns patients and dealing with delicate skin graft complications. Anticoagulant compounds in their saliva also show medicinal promise."

Source: Taxonomy Australia. (n.d.). Australia's Biodiversity: Segmented Worms (Phylum Annelida). Taxonomy Australia website. Available at: https://www.taxonomyaustralia.org.au/annelids [Accessed 11 July 2026]

Order: Sipuncula (Peanut worms)

Family: Antillesomatidae

–No subordinate taxa

Family: Aspidosiphonidae

–No subordinate taxa

Family: Golfingiidae

–No subordinate taxa

Family: Phascolosomatidae

–No subordinate taxa

Family: Siphonosomatidae

–No subordinate taxa

Family: Sipunculidae

–No subordinate taxa

Class: Clitellata

Subclass: Hirudinea (Leeches)

Order: Arhynchobdellida

Family: Americobdellidae

–No subordinate taxa

Family: Cyclobdellidae

–No subordinate taxa

Family: Cylicobdellidae

–No subordinate taxa

Family: Erpobdellidae

–No subordinate taxa

Family: Gastrostomobdellidae

–No subordinate taxa

Family: Orobdellidae

–No subordinate taxa

Family: Salifidae

–No subordinate taxa

Suborder: Hirudiniformes

Family: Haemadipsidae

–No subordinate taxa

Family: Haemopidae

–No subordinate taxa

Family: Hirudinidae (Medicinal leeches)

Family: Praobdellidae

–No subordinate taxa

Family: Semiscolecidae

–No subordinate taxa

Family: Xerobdellidae

–No subordinate taxa

Order: Rhynchobdellida

Family: Glossiphoniidae

–No subordinate taxa

Family: Ozobranchidae

–No subordinate taxa

Family: Piscicolidae

–No subordinate taxa

Subclass: Oligochaeta (Earthworms and their allies)

Superorder: Megadrili (Megadriles, including Earthworms)

Order: Megadrilaceae

Family: Glossoscolecidae

Family: Megascolecidae (Asian earthworms, Giant earthworms)

Order: Opisthopora

Family: Acanthodrilidae (incl. Octochaetidae)

Superorder: Microdrili (Microdriles)

–No subordinate taxa

Class: Polychaeta (Bristle or Polychaete worms, or Polychaetes) [paraphyletic]

Subclass: Echiura

Order: Echiuroidea

–No subordinate taxa

Subclass: Errantia

Order: Amphinomida

–No subordinate taxa

Order: Eunicida

Family: Eunicidae (Bloodworms, Palolo worms, Bobbit worms, Decorator worms)

Order: Phyllodocida

Family: Hesionidae

Family: Phyllodocidae (Paddle worms)

Subclass: Sedentaria

–No subordinate taxa

Superclass: Annelida incertae sedis

–No subordinate taxa

Statistics

Taxonomic Note

Take note that the phylum Annelida is undergoing rapid higher-level revision, and thus the taxonomic presentation here (an idiosyncratic selection based upon WoRMS and Glasby et al., 2025) is necessarily inaccurate:

"Annelida is a large phylum with approximately 20,000 species (Rouse et al. 2022; World Register of Marine Species (WoRMS) 2025) and is found in almost every terrestrial and aquatic habitat on Earth. For most of the last 170 years, Annelida have been divided into three majorclasses, Polychaeta (bristleworms), Oligochaeta (earthworms and allies) and Hirudinea (leeches). A fourth class, Archiannelida, containing a mix of minute polychaete-like annelids (Hermans 1969), now known to be unrelated, was rejected long ago by Fauchald (1977) but not finally abandoned until recent more taxon-specific phylogenomic studies firmly established their links with polychaete families containing large-bodied species (Andrade et al. 2015, and references therein). During the last century, Oligochaeta and Hirudinea were relegated to subclasses under class Clitellata Michaelsen, 1919 in recognition of a majorshared reproductive feature, the clitellum. Today, largely as a result of molecular studies, Clitellata is known to be deeply embedded within Polychaeta, which also includes the former phyla Sipuncula, Echiura, Pogonophora, and Vestimentifera (= Siboglinidae), meaning that ‘Polychaeta’ and ‘Annelida’ are almost one and the same concepts (Rouse et al. 2022, and references therein)."

Source: Glasby, Christopher J. et al. (2025). ANNiKEY Linear – diagnoses, descriptions, and a single-access identification key to Annelida family-level taxa. ZooKeys 1247: 217-403. https://doi.org/10.3897/zookeys.1247.137606

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