Belongs within: Eumetazoa.
Contains: Neonemertea.
The Nemertea, ribbon worms, are a phylum of mostly marine worms though there are also some freshwater and terrestrial species. Among the major features of the group is the presence of an eversible proboscis contained in an anterior chamber called the rhynchocoel. In basal members of the Nemertea, commonly classified in the past as the ‘Palaeonemertea’ though that grouping is probably paraphyletic, the proboscis is unarmed and the musculature relatively simple with a single layer of longitudinal muscles inside the outer circular muscle layer. The Cephalothricidae have a somewhat pointed head with the mouth positioned far back about four or five body widths from the anterior end.
Predatory ribbons
Published 29 April 2016
Some of you may have seen something like this doing the rounds:
The animal in the clip is called a nemertean. The Nemertea, commonly known as ribbon worms, are a group of more than 1200 known species of mostly predatory worm-like animals. The majority of nemerteans are marine, but there are also species found in freshwater or even terrestrial environments (the clip above shows a terrestrial species). As their vernacular name suggests, the majority of ribbon worms are flattish, slender animals with little in the way of external elaborations. Most are small and unassuming, but there are exceptions: one ribbon worm species from coasts of northern Europe, Lineus longissimus, grows to estimated lengths of over 30 m and may even be the longest animal in existence*. The most characteristic feature of ribbon worms is a long proboscis that they use in capturing prey; when not being deployed, this proboscis is retracted within a cavity called the rhynchocoel that runs much of the animal’s length. Other than this, nemerteans have little in the way of internal body cavities other than the gut. They do have a simple blood-vascular system consisting of a few blood vessels but no actual heart; instead, the blood just kind of sloshes back and forth as a result to the animal’s body contractions as it moves.
*Some uncertainty over the exact lengths of Lineus longissimus specimens is inevitable because, despite their remarkable length, they are still only a centimetre or so wide. When you’re trying to extract something like that from among a bunch of rocks, it’s gonna stretch and break. Still, thirty metres is a fairly conservative estimate of its length; Wikipedia cites a supposed maximum nearly twice that. These mega-nemerteans are definitely one of those animals that make me wonder, how does this thing even exist? I mean, what is the point of being so incredibly long and slender? How does it collect enough food at the front end to nourish itself all the way to the back end? How does it not just fall apart of its own accord, let alone when subjected to any external pressure?
The relationships of nemerteans to other animals are rather uncertain, and they have generally been classified as their own independent phylum. Because of their simple body plan, many early authors compared them to flatworms, at least on a grade level, but this fell out of favour as it became accepted that the rhynchoel and blood-vascular system probably correspond to anatomical structures in more complex animals. More recent evidence from molecular and other sources has converged on a position within the Lophotrochozoa, the major animal clade that also includes molluscs, brachiopods and annelids, but their exact placement within this clade remains open to debate.
Molecular data have also influenced our understanding of relationships within the Nemertea. An influential classification of the group divided them between the Enopla, in which the proboscis is usually armed with a stabbing stylet or stylets, and the Anopla, in which the proboscis is unarmed (members of this latter group often have the proboscis branched as in the clip above; I’m guessing that in the absence of a stylet the proboscis probably works through adhesion). The two groups also differ in that Anopla always have the proboscis emerging from a separate pore to the mouth, whereas in many Enopla the mouth and proboscis pore share a common opening (Kvist et al. 2014). The Anopla were further subdivided into the Heteronemertea, which have a distinctive tissue layer called the dermis underneath the outer epidermis, and the Palaeonemertea which lack such a differentiation of skin layers. However, one need not be an expert in nemerteans to spot that the Anopla and Palaeonemertea were mostly defined by their lack of derived features (no stylets, no dermis) and so it should come as little surprise that molecular studies of the group have failed to offer resounding support for their monophyly. Instead, a number of studies have suggested that the Heteronemertea and Enopla together form a clade that Thollesson & Norenburg (2003) dubbed the Neonemertea. When they did so it was on the basis of molecular data only, but later authors have identified possible synapomorphies of the Neonemertea in features of the nervous and blood-vascular systems. One family of ‘Palaeonemertea’, the Hubrechtidae, has been suggested to also belong within the Neonemertea as sister-taxon to the Heteronemertea. This is of interest because the Hubrechtidae and Heteronemertea share a distinctive type of ciliated planktonic larva called a pilidium (other nemerteans either develop directly or have a creeping planula-type larva). Ciliated planktonic larvae are known a number of groups of animals, such as the veliger of molluscs, the trochophore of annelids, or the tornaria of acorn worms, and there has been a lot of discussion over the years as to whether similarities between these larvae represent a shared ancestry, or whether they might have evolved independently. In the case of nemerteans, at least, the current evidence seems to favour the latter. As for the other ‘palaeonemerteans’, there seems to be less of a consensus as to whether they form a single clade or a paraphyletic series relative to the Neonemertea.
As for the Enopla, it appears to form a valid clade. Previous authors divided the enoplans between the Hoplonemertea, including the majority of species, and the Bdellonemertea, including the single distinctive genus Malacobdella. The Hoplonemertea were in turn divided between the Monostilifera, in which the proboscis has a single long stylet, and the Polystilifera, in which it bears a pad of small stylets, and molecular analyses support the separation of these groups. Malacobdella (which lacks proboscis stylets but has the conjoined mouth-proboscis pore) has a sucker at the posterior end of its body, by which it lives attached to the gills of a mollusc. Malacobdella is not a parasite of the mollusc, per se: instead, it feeds on food particles drawn in by water flowing through the mollusc’s gills. However, the recent analyses have indicated that Malacobdella is in fact a derived monostiliferan, and a number of recent authors have used the Hoplonemertea as an equivalent name to the old Enopla.
Also distinctive within the Hoplonemertea are two clades, the polystiliferan Pelagica and the monostiliferan Korotkevitschiidae, that have left the ocean floor and adopted a pelagic life style. Members of both these groups are gelatinous and eyeless; the Pelagica have lost further internal organs such as nephridia. The Korotkevitschiidae (which also lack a proboscis stylet) are found towards the surface of the ocean; the Pelagica are found in much deeper waters (Chernyshev 2003b). The pelagic nemerteans are among the most poorly known of all ribbon worms; they are rarely encountered (about half of the 100 or so described species of Pelagica are known only from single specimens) and their relatively simple morphology makes them difficult to compare to other nemerteans. If the individual in the photograph is any indication, however, they are beautiful animals.
Systematics of Nemertea
Characters (from Gibson 2002): Epidermis ciliated. Ciliated alimentary tract with separate mouth and anus. Blood system closed, composed of distinct vessels or lacunae. Eversible muscular proboscis present, housed (when retracted) in fluid-filled tubular chamber (rhynchocoel) extending posteriorly above gut. Nervous system well developed, with paired lateral longitudinal nerve cords extending from lobed cerebral ganglia. Excretory system usually protonephridial.
<==Nemertea [Anopla, Nemertina, Nemertini, Palaeonemertea, Rhynchocoela] |--Archisymplectes rhothon Schram 1973EL11, W93 `--+--Carinina arenaria Hylbom 1957TN03, K86 `--+--+--NeonemerteaTN03 | `--Carinoma [Carinomidae]TN03 | |--C. mutabilis Griffin 1898TN03 | `--C. tremaphoros Thompson 1900TN03 `--+--TubulaniformesC03 | |--ParahubrechtiaC03 | `--Tubulanus [Tubulanidae]TN03 | |--T. annulatus (Montagu 1804)RP07 | |--T. polymorphusPP15 | |--T. punctatus (Takakura 1898)TN03 | |--T. rhabdotus Corrêa 1954TN03 | `--T. sexlineatus (Griffin 1898)TN03 `--Cephalothricidae [Archinemertea]TN03 |--CephalotrichellaG02 |--ProcephalothrixTN03 | |--P. filiformis (Johnston 1828)TN03 | |--P. kiliensis Friedrich 1935K86 | |--P. simulus Iwata 1952TN03 | `--P. spiralis (Coe 1930)TN03 `--Cephalothrix Oersted 1844K86 |--C. arenaria Hylbom 1957K86 |--C. atlantica Gerner 1969K86 |--C. bipunctataPP64 |--C. germanica Gerner 1969K86 |--C. hongkongiensisCV16 |--C. linearisPP64 |--C. mediterranea Gerner 1969K86 |--C. pacifica Gerner 1969K86 `--C. rufifronsPP64 Nemertea incertae sedis: Arhynchonemertes Riser 1988G02 `--*A. axi Riser 1988G02 Carinesta tubulanoides Gibson 1990G02 Hirudella Münster 1842H62 `--*H. angusta Münster 1842H62 Legnodesmus Ehlers 1869H62 `--*L. ehlersi Howell 1958H62 Planolineus exsulG88 Siolineus turbidusG88 Apatronemertes albimaculosaG88 NemertesG88 |--N. carcinophilaM01 `--N. polyhopla (n. d.)G88 BorlasiaM01 |--B. elisabethaeM01 `--B. marina [=Gordius marinus]G20
*Type species of generic name indicated
References
[CV16] Cannon, J. T., B. C. Vellutini, J. Smith, III, F. Ronquist, U. Jondelius & A. Hejnol. 2016. Xenacoelomorpha is the sister group to Nephrozoa. Nature 530: 89–93.
[C03] Chernyshev, A. V. 2003a. New species of the genus Hubrechtella (Nemertea, Anopla) from the Sea of Japan and validation of the family Hubrechtellidae. Russian Journal of Marine Biology 29: 333–336.
Chernyshev, A. V. 2003b. Classification system of the higher taxa of enoplan nemerteans (Nemertea, Enopla). Russian Journal of Marine Biology 29 (Suppl. 1): S57–S65.
[EL11] Erwin, D. H., M. Laflamme, S. M. Tweedt, E. A. Sperling, D. Pisani & K. J. Peterson. 2011. The Cambrian conundrum: early divergence and later ecological success in the early history of animals. Science 334: 1091–1097.
[G02] Gibson, R. 2002. The Invertebrate Fauna of New Zealand: Nemertea (ribbon worms). NIWA Biodiversity Memoir 118. National Institute of Water and Atmospheric Research: Wellington.
[G20] Goldfuss, G. A. 1820. Handbuch der Naturgeschichte vol. 3. Handbuch der Zoologie pt 1. Johann Leonhard Schrag: Nürnberg.
[G88] Gray, J. 1988. Evolution of the freshwater ecosystem: the fossil record. Palaeogeography, Palaeoclimatology, Palaeoecology 62: 1–214.
[H62] Howell, B. F. 1962. Worms. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W144–W177. Geological Society of America, and University of Kansas Press.
[K86] Kirsteuer, E. 1986. Nemertina. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 72–75. E. J. Brill/Dr W. Backhuys: Leiden.
Kvist, S., C. E. Laumer, J. Junoy & G. Giribet. 2014. New insights into the phylogeny, systematics and DNA barcoding of Nemertea. Invertebrate Systematics 28: 287–308.
[M01] M’Intosh, W. C. 1901. The coloration of marine animals. Annals and Magazine of Natural History, series 7, 7: 221–240.
[PP64] Peres, J. M., & J. Picard. 1964. Nouveau manuel de bionomie benthique de la mer Mediterranee. Recueil des Travaux de la Station Marine d’Endoume, Bulletin 31 (27): 5–137.
[PP15] Pisani, D., W. Pett, M. Dohrmann, R. Feuda, O. Rota-Stabelli, H. Philippe, N. Lartillot & G. Wörheide. 2015. Genomic data do not support comb jellies as the sister group to all other animals. Proceedings of the National Academy of Sciences of the USA 112 (50): 15402–15407.
[RP07] Rousset, V., F. Pleijel, G. W. Rouse, C. Erséus & M. E. Siddall. 2007. A molecular phylogeny of annelids. Cladistics 23: 41–63.
[TN03] Thollesson, M., & J. L. Norenburg. 2003. Ribbon worm relationships: a phylogeny of the phylum Nemertea. Proceedings of the Royal Society of London Series B—Biological Sciences 270: 407–415.
[W93] Wills, M. A. 1993. Miscellania. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 555–560. Chapman & Hall: London.