Segmented Worms (Phylum Annelida)
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.
Somewhere between one- and two-thirds of the Australian annelid fauna remain undescribed, including this species in the genus Myrianida (Syllidae) from the shallow waters of Port Phillip Bay, Victoria. Photo: (c) Andrew Newton
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.
A blood-feeding terrestrial leech. Leeches are annelids; this one clearly shows the multiple segments making up the worm-shaped body. There are around 60 named species of terrestrial, freshwater and marine leeches in Australia, but no active leech taxonomists.
Image: Anna D'Accione. Source: https://australianmuseum.net.au/learn/animals/worms/leeches/
Around 23,000 species of annelids have been described globally; the best estimate is that between one-third and two-thirds of species remain undiscovered or un-named. Around 2,600 species are currently known from Australia; the annelid fauna of Australia is likely to be between 4,000 and 8,000 species.
World-wide about 150 new species are described every year, with on average ten per year from Australia and its surrounding oceans. Unless the taxonomic discovery and documentation of annelids is substantially increased, they will remain poorly known for many decades.
Many marine annelids are spectacular in life, like this cirratulid deposit feeder from Hobsons Bay, Melbourne. The family Cirratulidae is common and diverse in most marine environments but have never been studied systematically in Australian waters. It is likely that this is another undescribed species. Image: David Paul. Source: Museums Victoria CC-BY-NC
Collections of annelids are held in all Australian natural history museums. Australia has more than 15 active annelid taxonomists but the majority have emeritus and honorary associations with museums, or are students and postgraduates with limited tenure.
Read more about annelids in Australia: