Hellenica World

Echiura

Atlantic echiurans or spoon worms

Atlantic echiurans or spoon worms

Cladus: Eukaryota
Supergroup: Opisthokonta
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Protostomia
Cladus: Spiralia
Cladus: Lophotrochozoa
Phylum: Echiura
Ordines: Echiuroidea - Heteromyota - Xenopneusta

References

* Biseswar, R. 2009: The geographic distribution of echiurans in the Atlantic Ocean (Phylum Echiura). Zootaxa, 2222: 17-30. Abstract & excerpt
* Chipman, A.D. 2008: Annelids step forward. Evolution & development, 10(2): 141-142.
* Gordon, D.P. (ed.) 2009: New Zealand inventory of biodiversity. Volume 1. Kingdom Animalia. Radiata, Lophotrochozoa, Deuterostomia. Canterbury University Press, Christchurch, New Zealand.
* Hessling, R.; Westheide, W. 2002: Are Echiura derived from a segmented ancestor? Immunohistochemical analysis of the nervous system in developmental stages of Bonellia viridis. Journal of morphology, 252(2): 100-113.
* McHugh, D. 1997: Molecular evidence that echiurans and pogonophorans are derived annelids. PNAS, 94: 8006–8009.
* Nishikawa, T. 1998: Nomenclatural remarks on the family-group names of the phylum Echiura. Proceedings of the Biological Society of Washington, 111: 249-256.
* Rouse, G.W.; Pleijel, F. 2007: Annelida. Pp. 245-264 in: Zhang, Z.-Q. & Shear, W.A. (eds.) Linnaeus tercentenary: progress in invertebrate taxonomy. Zootaxa, 1668: 1–766. PDF
* Struck, T.H.; Schult, N.; Kusen, T.; Hickman, E.; Bleidorn, C.; McHugh, D.; Halanych, K.M. 2007: Annelid phylogeny and the status of Sipuncula and Echiura. BMC evolutionary biology, 7: 57. [1]

Eastern Pacific echiurans or spoon worms

Eastern Pacific echiurans or spoon worms

Vernacular names
Deutsch: Igelwürmer
English: burrow worms, spoon worms
Français: Échiuriens, Bonellies
Hrvatski: Zvjezdani
Magyar: ormányosférgek
Македонски: Лажичести црви
日本語: ユムシ動物門
Polski: Szczetnice

The Echiura, or spoon worms, are a small group of marine animals. They are often considered to be a group of annelids, although they lack the segmented structure found in other members of that group, and so may also be treated as a separate phylum. However, phylogenetic analyses of DNA sequences place echiurans and pogonophorans within the Annelida.[2] The Echiura fossilise poorly and the earliest known specimen is from the Upper Carboniferous (called the Pennsylvanian in North America). However, U-shaped fossil burrows that could be Echiuran have been found dating back to the Cambrian.

Echiurans are marine worms similar in size and habit to sipunculans. Many genera, such as Echiurus, Urechis, and Ikeda, live in burrows in sand and mud; others live in rock and coral crevices. One species, Thalassema mellita, which lives off the southeastern coast of the US, inhabits the tests (exoskeleton) of dead sand dollars. When the worm is very small, it enters the test and later becomes too large to leave.

The majority of echiurans live in shallow water, but there are also deep sea forms. About 140 species have been described.

Anatomy
Echiurans have a worm-like body with a large flattened proboscis projecting forward from the head. The body is typically drab in colour, but bright red and green species are known. The proboscis is a sheet-like structure, rolled around into a cylindrical tube with an open gutter at the ventral surface. The length of the proboscis varies greatly between species, and in some species is many times longer than the rest of the body. It is probably homologous with the prostomium of other annelids.[3]

Compared with other annelids, echiurans have relatively few setae. In most species, there are just two, located on the underside of the body just behind the proboscis. In others, such as Echiurus, there are also further setae near the posterior end of the animal. Unlike other annelids, adult echiurans have no trace of segmentation.[3]

The digestive system consists of a simple tube running the length of the body, with the anus being at the posterior end. The tube, however, is highly coiled, giving it a considerable length in relation to the size of the animal. A pair of simple or branched diverticula are connected to the rectum. These are lined with numerous minute ciliated funnels that open directly into the body cavity, and are presumed to be excretory organs.[3]

Although some species lack a blood vascular system, where it is present, it resembles that of other annelids. The blood is essentially colourless, although some haemoglobin-containing cells are present in the coelomic fluid of the main body cavity. There can be anything from one to over a hundred metanephridia for excreting nitrogenous waste, which typically open near the anterior end of the animal.[3]

Echiruans do not have a distinct respiratory system, absorbing oxygen through the body wall.

The nervous system consists of a brain near the base of the proboscis, and a ventral nerve cord running the length of the body. Aside from the absence of segmentation, this is a a similar arrangement to that of other annelids. Echiurans do not have any eyes or other distinct sense organs.[3]
Feeding

Typical spoon worms, including Bonellia, are suspension feeders, projecting their proboscis out of their burrows, with the gutter projecting upwards. Edible particles will then settle onto the proboscis and a ciliated channel conducts the food to the trunk.

Perhaps the most remarkable feeding adaptations among the spoon worms can be seen in Urechis. U. caupo lives in a large, U-shaped burrow and by pulsating its body it drives water through its lair. To feed, it produces a conical mucus net that lines the burrow as water is sucked in at a rate of about 18L per hour. Edible particles are caught on the net, and after some time the worm slowly eats the net and all the edible matter sticking to it.[4]
Reproduction

Echiruans are dioecious, with separate male and female individuals. The gonads are associated with the peritoneal membrane lining the body cavity, into which they release the gametes. The sperm and eggs complete their maturation in the body cavity, before being released into the surrounding water through the metanephridia. Fertilisation is external.[3]

The species Bonellia viridis, also remarkable for the possible antibiotic properties of bonellin, the green chemical in its skin, is unusual for its extreme sexual dimorphism. Females are typically 8 centimetres (3.1 in) in body length, excluding the proboscis, but the males are only 1 to 3 millimetres (0.039 to 0.12 in) long, and spend their lives within the uterus of the female.[3]

Fertilized eggs hatch into free-swimming trochophore larvae. In some species, the larva briefly develops a segmented body before transforming into the adult body plan, supporting the theory that echiruans evolved from segmented ancestors resembling more typical annelids.[3]
Notes and references

1. ^ Name authority
2. ^ Struck, TH; et al., N; Kusen, T; Hickman, E; Bleidorn, C; McHugh, D; Halanych, KM (2007-05-27). "Annelid phylogeny and the status of Sipuncula and Echiura". BMC Evolutionary Biology (BioMed Central) 7 (57): 57. doi:10.1186/1471-2148-7-57. PMC 1855331. PMID 17411434. http://www.biomedcentral.com/1471-2148/7/57/abstract
3. ^ a b c d e f g h Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 870–873. ISBN 0-03-056747-5.
4. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Publishing Group.

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