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Segmented Worms - Phylum: Annelida
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"A man may fish with a worm that hath eat of a king, and eat of a fish that hath fed of that worm.”
Prince Hamlet in 'Hamlet', Act 4, Scene 3, by William Shakespeare (1564-1616).
Prince Hamlet in 'Hamlet', Act 4, Scene 3, by William Shakespeare (1564-1616).
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Also known as Ringed Worms, the Annelids are typically divided into two classes, the Clitellata (Clitellates) and the Polychaeta (Bristle, or Polychaete Worms). The former hosts the sub-classes of the Hirudinea (Leeches) and the Oligochaeta (Earthworms and Allies). The latter is home of the Spoon Worms and Bristleworms. Altogether the Annelids account for over 22,000 species.
They can be found in aquatic environments of any variety; tidal zones, rivers and creeks, reefs, mangroves, submerged artificial structures, even hydrothermal vents. There are also numerous species that live exclusively on land. Recent studies have shown that the previously recognized phylla: Pogonophora and the Echiura, as well as the phyla: Sipuncula, Annelida, Brachiopoda, Mollusca and Nemertea are to be regarded as parts of a large Superphylum called Lophotrochozoa. Some member phyla are still treated separately here. |
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Other than the above-mentioned classes iNaturalist shows this phylum to also host the order: Myzostomida, which are a diverse group of small worms that are parasitic on Echinoderms, usually Crinoids. Taxonomic placement of this group of animals has been uncertain and as a consequence they have been placed in various phyla over the years including Acanthocephala, Tardigrada, Rotifera and Platyhelminthes. Only of late are they considered to be part of the Annelida although there is little concensus about their actual place within that phylum.
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Whereas Myzostomids come in a varied range of sizes and shapes they are commonly flat, rounded animals with gradually thinner edges from which fine short hairs protrude.
There are also three families of small to microscopic worms some of which are only recently considered to be members of the phylum: Annelida. Species of these families and the Myzostomids are usually only encountered by people who are specifically searching for them and even though some may well occur in the Northern Rivers area they are not dealt with here any further. |
Image 1.
Tubeworms near hydrothermal vent.
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In general, most Annelids consist of multiple segments each of which has the same set of organs and, often, as in the case of the Polychaeta, a set of parapodia which are structures used for mobility. The segments are usually separated by septa (=dividing walls) but in many species these are not well defined or even absent. The septa allow the animal to change the shape of the segments and when done in sequence this in turn enables the animal to move in a forward direction in a rippling motion called peristalsis. Some species that have parapodia use an undulating sequence making the use of the parapodia more effective.
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Due to the Annelids being soft-bodied there is little evidence of them in the fossil record in general but, every now and then fossils of what appear to be Annelids do surface. From these rare occurrences it has been tentatively determined that the first Annelids date from around 518 million years ago. These fossils were found in Greenland.
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At a point around 300 million years ago the class: Polychaeta had reached most of the diversification currently on show. The Oligochaeta were not clearly present until the Tertiary period, around 65 million ago, although earthworms may have been present in the Triassic period around 250 million years ago.
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References and links:
> Image 1. NOAA Okeanos Explorer Program, Galapagos Rift Expedition 2011 Source: Flickr NOAA Photo Library Open source image downloaded for use here 02/08/2022.
> Ruppert, E.E., Fox, R.S. and Barnes, R.D., (2004). 'Invertebrate Zoology: A functional and evolutionary approach.' (7th Ed.) Brooks/Cole-Thomson Learning, Belmont, CA, U.S.A.
https://en.wikipedia.org/wiki/Hydrothermal_vent
https://en.wikipedia.org/wiki/Parapodium
https://en.wikipedia.org/wiki/Peristalsis
https://inaturalist.ala.org.au/observations?taxon_id=47491
https://bie.ala.org.au/species/https://biodiversity.org.au/afd/taxa/1f5a5c77-fcef-45b9-84ad-bfea95fd1e62
Header photo: