Nemertea
| Nemertea | |
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| Parborlasia corrugatus from the Ross Sea | |
| Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Superphylum: | Lophotrochozoa |
| Phylum: | Nemertea Schultze, 1851 |
| Classes | |
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Anopla |
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| Synonyms | |
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Rhyncocoela [1] |
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Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms.[1] Most of the 1,400 or so species are marine, with a few living in fresh water and a small number of terrestrial forms; they are found in all marine habits, and throughout the world's oceans.[2] Nemerteans are named after Nemertes, one of the Nereids of Greek mythology, and alternative spellings for the phylum have included Nemertini and Nemertinea. Libbie Hyman named them Rhynchocoela, a name used primarily in North America but gradually abandoned since the 1980s.
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History
The earliest record of a nemertean worm is probably an account by Olaus Magnus in 1555 of a long, greyish-blue marine worm, which is probably Lineus longissimus, but the first species was not formally described until Gunnerus described the same species (as Ascaris longissima) in 1770.[3] In 1995, a total of 1,149 species had been described and grouped into 250 genera.[3]
Ecology and distribution
The majority of nemertean worms live on or in the sea floor, with many species extending into brackish water in estuaries, and some freshwater or fully terrestrial species. They are often found in and among seaweeds, rocks, mussel and barnacle beds, or buried in mud, sand, or gravel substrates. Freshwater genera include the large genus Prostoma, while the terrestrial forms are best represented by Geonemertes, a genus mostly found in Australasia, but with one species in the Seychelles, one found widely across the Indo-Pacific, one from Tristan da Cunha in the South Atlantic, and one, G. chalicophora, first found in the Palmengarten in Frankfurt, but since discovered in the Canary Islands, Madeira and the Azores.[3]
Most nemerteans are carnivorous and predatory, catching prey with their proboscis,[4] although some are scavengers and some are herbivores.[5] In some families, it is armed with a sharp stylet which may be poisonous, while those that lack the stylet often use a sticky secretion on the proboscis to entrap their prey. The proboscis is wrapped around the prey, which is normally other invertebrates such as crustaceans and annelids and can be many times larger than the nemertean itself, and the prey is then stabbed repeatedly with the stylet until dead.[6] A few, such as Malacobdella, live commensally in the mantle cavity of molluscs by "stealing" from the food filtered by the host. [6]
Anatomy
Nemertean worms are long, thin, unsegmented animals. They have no true head, although the anterior end is often slightly wider than the body. They are distinguished by the presence of an eversible proboscis which is used for catching prey. Although generally considered acoelomate, the cavity which contains the proboscis includes a true coelom.[7] The body surface is ciliated, which may be the primary source of propulsion for the smaller species. Larger nemerteans use muscular peristalsis to crawl or swim.
Respiration is entirely by diffusion.[4]
The receptor of the nemertine worms have a similar structure, but has a rootlet the function of which is not known, Moreover, the rhabdomere are not found to bear or originate from cilia.[8]
Length
Nemerteans range in size from 5 millimetres (0.20 in) to over 30 metres (98 ft) long in the case of the European Lineus longissimus, with most species being 20 centimetres (8 in) or less. There are also reports of specimens up to 50 m or 60 m long, which would make it the longest animal in the world;[9] the longest vertebrate on record is a female blue whale, 29.9 m long.[10]
Digestive system
Unlike flatworms, which have a single opening, the nemertean digestive system includes a separate mouth and anus. In most species, the mouth is located on the underside of the animal, a short distance back from the anterior end. It opens into a buccal cavity, an oesophagus, and then a stomach lined with cells secreting digestive enzymes. Beyond the stomach lies an intestine that takes up most of the length of the body, and has numerous diverticula.[11]
Nemerteans collect food with their proboscis, which is closely associated with the digestive system, although it actually develops from the ectoderm on the outside of the animal. At rest, the proboscis lies within a long tube that may take up a considerable portion of the worm's length, lying just above the gut. It is attached to the posterior end of this tube by a muscle that pulls it back inside after feeding. At the anterior end, the tube opens into a small cavity close to the brain, and then to the outside through a pore at the anterior tip of the animal.[11]
Circulatory system
The circulatory system of nemerteans is closed, with all of the vessels having a distinct lining. In the many nemerteans, such as Cephalothrix, the system consists only of two parallel blood vessels, one lying on either side of the gut, and connected at either end of the animal by slightly wider spaces called lacunae. In other species, this basic plan has been elaborated by the addition of a third vessel running dorsal to the gut, and by various transverse vessels connecting these three together.[11]
The lateral blood vessels are each associated with a single protonephridium, which in some cases, even projects directly into the blood. This opens through a pore near the anterior end of the animal, level with foregut. In many species, the original protonephridium has become split into multiple separate tubules, each with its own pore. These structures remove waste products from the blood, acting as a primitive excretory system.[11]
Although the blood vessels themselves have muscular linings, nemerteans do not possess a heart. Contraction of the vessels and movements of the body muscles can cause the blood to move through the vessels, but there is no preferred direction. The blood itself generally lacks respiratory pigments, but may contain phagocytic amoebocytes or other corpuscles.[11]
Nervous system
The nervous system includes a brain and several nerve cords. The brain consists of four ganglia connected in a ring around the oesophagus. From here, two main nerve cords, one on each side, run down the length of the animal, with numerous smaller nerves branching out from them. In the most primitive nemerteans these lie within the skin, but in most they are deeper, inside the muscle layers. Most nemerteans also have additional, smaller, nerve cords, typically including one running down the midline of the dorsal surface, and two ennervating the oesophagus and stomach.[11]
Sensory nerve endings are distributed across the body, but a number of special sense organs are found near the anterior end of the animal. Most nemerteans have two to six eyes, although some have hundreds. These are relatively simple structures, consisting of pits lined by photoreceptor cells, similar to those in flatworms. Narrow grooves in the head region are lined with cilia and have numerous nerve endings; they are probably chemoreceptors, used to taste the surrounding water.[11]
Nemertean worms are unique in possessing a pair of "cerebral organs" — sensory and regulatory organs closely associated with the brain.[2] These consist of pores in the upper surface of the head, opening into narrow canals lined by cilia. Numerous sensory cells - most likely chemoreceptors - line the ends of the pits, as do glandular cells connected with the brain.[11]
Reproduction and life cycle
Nemerteans often have numerous gonads, forming a row down each side of the body. Most species have separate sexes, although all the freshwater forms are hermaphroditic. Fertilization is usually external, although some bathypelagic species have internal fertilization, and, in a few cases, also live birth.[2] Gametes are extruded from the body by muscular action through temporary pores that form in each gonad.
In most nemerteans, the eggs hatch into smaller versions of the adult, but some have a pilidium larva. These are somewhat conical in form, with a pair of lobes hanging down from the underside and a tuft of elongated cilia at the upper pole.[11]
The species Paranemertes peregrina has been reported as having a life span of around 18 months.[11]
Classification
The fossil record of the phylum is sparse, as expected for a group of soft-bodied animals, but even the hard stylets are not found. The only possible nemertean fossil is Archisymplectes from the Mazon Creek biota of the Pennsylvanian of Illinois.[6]
Once classified as "degenerate" flatworms, nemerteans are now recognised as a separate phylum, more closely related to higher, coelomate phyla in Lophotrochozoa, such as Annelida and Mollusca.[12] The sequence of the genome of the mitochondrion of Cephalothrix simula places this phylum closer to the coelomate lophotrochozoans rather than the acoelomate platyhelminths consistent with earlier work.[13]
The traditional classes of Enopla for nemerteans armed with one or more stylets and Anopla for those without are not monophyletic is not supported by molecular data.[14] Similarly, the subclass Bdellonemertea, erected for nemerteans which live as parasites on molluscs, is nested within Hoplonemertea, and probably represents a specialised offshoot from that group rather than an independent lineage [14]. Recent molecular phylogenetic study has, however, confirmed the monophyly of each of Heteronemertea and Hoplonemertea, as well as the expected paraphyly of Palaeonemertea.[15]
References
- ↑ 1.0 1.1 "Nemertea". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=57411.
- ↑ 2.0 2.1 2.2 J. Moore & R. Gibson (2001). Nemertea. Encyclopedia of Life Sciences. pp. 4 pp. doi:10.1038/npg.els.0001586.
- ↑ 3.0 3.1 3.2 R. Gibson (1995). "Nemertean genera and species of the world: an annotated checklist of original names and description citations, synonyms, current taxonomic status, habitats and recorded zoogeographic distribution". Journal of Natural History 29 (2): 271–561. doi:10.1080/00222939500770161.
- ↑ 4.0 4.1 "Nemertea (ribbon worms)". bumblebee.org. http://www.bumblebee.org/invertebrates/NEMERTEA.htm. Retrieved August 10, 2007.
- ↑ Stephen Shaner. "Phylum Rhyncocoela". Seamuse.org. http://www.seamuse.com/rhyncocoela.htm. Retrieved August 10, 2007.
- ↑ 6.0 6.1 6.2 Ben Waggoner & Allen G. Collins (June 13, 2001). "Introduction to the Nemertini". University of California, Berkeley. http://www.ucmp.berkeley.edu/nemertini/nemertini.html.
- ↑ J. McClintock Turbeville, K. G. Field & R. A. Rafl (1992). "Phylogenetic position of Phylum Nemertini, inferred from 18s rRNA sequences: molecular data as a test of morphological character homology". Molecular Biology and Evolution 9 (2): 235–249. http://mbe.oxfordjournals.org/cgi/reprint/9/2/235.
- ↑ "Photoreception."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD. 2009.
- ↑ Kåre Telnes. "Giant ribbon worm". The Marine Fauna Gallery of Norway. http://www.seawater.no/fauna/slimormer/kjempe.htm. Retrieved 2007-08-10.
- ↑ "COSEWIC Assessment and Update Status Report on the Blue Whale Balaenoptera musculus in Canada". Committee on the Status of Endangered Wildlife in Canada. Ottawa. 2002. http://www.wildwhales.org/cetaceans/blue/sr_blue_whale_e.pdf.pdf.
- ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 252–262. ISBN 0-03-056747-5.
- ↑ "Bilateria". Tree of Life Web Project. 2002-01-01. http://www.tolweb.org/Bilateria/2459.
- ↑ H. X. Chen, P. Sundberg, J. L. Norenburg & S. C. Sun (2009). "The complete mitochondrial genome of Cephalothrix simula (Iwata) (Nemertea: Palaeonemertea)". Gene 442 (1–2): 8–17. doi:10.1016/j.gene.2009.04.015.
- ↑ 14.0 14.1 Per Sundberg, J. M. Turbeville and Susanne Lindh (2001). "Phylogenetic relationships among higher nemertean (Nemertea) taxa inferred from 18S rDNA sequences". Molecular Phylogenetics and Evolution 20 (3): 327–334. doi:10.1006/mpev.2001.0982.
- ↑ Mikael Thollesson and Jon L. Norenburg (2003). "Ribbon worm relationships: a phylogeny of the phylum Nemertea". Proceedings of the Royal Society of London B 270: 407–415. doi:10.1098/rspb.2002.2254.
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