Spoonworms - Subclass: Echiura
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The Prussian natural-historian, Peter Simon Pallas, in 1776, was the first to describe and name a Spoonworm but he mistakenly took it for a species belonging to the Genus: Lumbricus as he thought it to be a kind of earthworm. After a handful of re-shuffles, including a classification as a distinct Phylum, Spoonworms eventually (after 2011) became the Subclass they are now. Despite being placed in the Phylum: Segmented Worms, Spoonworms do not have segments. It is generally considered that they lost the segmentation secondarily (= present originally but lost during evolution over time). Fossils of Echiurids are very rare, the oldest fossil that is definitely that of a Spoonworm dates from the Upper Carboniferous (323 - 299 Mya). It was found in the Mazon Creek fossil beds near Morris in Illinois, U.S.A.
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Spoonworms are unusual creatures and possibly best described as thick sausages, somewhat thinner in the middle, with a large extendable proboscis protruding from the head just underneath the mouth. This proboscis sometimes has a scoop at the end, sometimes it is forked and, in many species can be quite out of proportion to the size of the body. The smallest species do not get much longer than one cm whereas the longest can exceed two meters in length with the proboscis fully extended. There are currently only some 175 species of Spoonworms but they can be found in marine benthic habitats worldwide. Most prefer to burrow - body first - into the sediments in shallow water. Some do not burrow, preferring to shelter underneath rocks or other bottom structures. And, there are quite a few deep-sea species as well.
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The proboscis on Echiurids seems to serve a number of functions. For locomotion it extends the proboscis forward - using cilia on the lower surface - until it finds something to grab hold of and than pulls itself forward. To feed itself it uses the proboscis to gather detritus and channel it towards the mouth. To breathe, oxygen passes through the skin of both body and proboscis. Although, interestingly, some species breath by means of cloacal irrigation, which involves pulling water in through the anus to allow oxygen to be absorbed by the intestinal wall prior to expelling the water again.
Spoonworms sometimes wash up on the beach and in some East Asian countries are actively searched for by the locals who dig them up from their burrows and sell them at market as food. The Innkeeper Worm, aka Penis Worm, Urechis unicinctus, in particular, is highly prized. |
Suborder: Bonelliida
Family: Bonelliidae
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Metabonellia sp.
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Family: Ikedidae
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Ikeda sp.
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Suborder: Echiurida
Family: Echiuridae
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Echiurus sp.
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Family: Thalassematidae
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Anelassorhynchus sp. (likely occurrence)
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Arhynchite sp.
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Listriolobus sp.
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Ochetostoma sp.
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Thalassema sp.
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Taxonomy:
Phylum: Annelida (Segmented Worms)
> Class: Polychaeta (Polychaete Worms)
> Subclass: Echiura (Spoonworms)
> Order: Echiuroidea (Spoonworms)
> Suborder: Bonelliida
> Family: Bonelliidae
> Genus: Metabonellia
> Family: Ikedidae
> Genus: Ikeda
> Suborder: Echiurida
> Family: Echiuridae
> Genus: Echiurus
> Family: Thalassematidae
> Genus: Anelassorhynchus *
> Genus: Arhynchite
> Genus: Listriolobus
> Genus: Ochetostoma
> Genus: Thalassema
* = likely
References and links:
> Pallas, P.S., (1766). 'Miscellanea zoologica. Quibus novae imprimis atque obscurae animalium species describuntur et observationibus iconibusque illustrantur.' Petrum van Cleef. Hagí Comitum., xii + 224 pp.;14 pls. (no publishers' location; possibly Utrecht, Netherlands), https://archive.org/details/pspallasmedicina00pall (PP 146+, Plate XI, figures 1-6.)
https://en.wikipedia.org/wiki/Mazon_Creek_fossil_beds
https://onlinezoologists.com/cs/echiura
https://en.wikipedia.org/wiki/Peter_Simon_Pallas
https://en.wikipedia.org/wiki/Urechis_unicinctus
> Pallas, P.S., (1766). 'Miscellanea zoologica. Quibus novae imprimis atque obscurae animalium species describuntur et observationibus iconibusque illustrantur.' Petrum van Cleef. Hagí Comitum., xii + 224 pp.;14 pls. (no publishers' location; possibly Utrecht, Netherlands), https://archive.org/details/pspallasmedicina00pall (PP 146+, Plate XI, figures 1-6.)
https://en.wikipedia.org/wiki/Mazon_Creek_fossil_beds
https://onlinezoologists.com/cs/echiura
https://en.wikipedia.org/wiki/Peter_Simon_Pallas
https://en.wikipedia.org/wiki/Urechis_unicinctus
Photographic contributions: