Sagitta maxima, from here.

Belongs within: Gnathifera.
Contains: Aidanosagitta.

Chaetognatha, arrow worms, are small marine animals from 2 to 120 mm in length. Superficially, they are somewhat fish-like, with an elongate body bearing lateral and terminal fins, despite not being closely related to vertebrates. The actual relationships of chaetognaths have long been contentious, and even now have not been firmly established though they probably hold a reasonably basal position within the protostomes. The head of chaetognaths bears a number of grasping chitinous spines covered in rest by a hood that can be extended for capturing prey. The majority of chaetognaths are planktonic though some, such as the genus Spadella, are benthic.

The Protoconodontida, Cambrian-Ordovician phosphatised spine-shaped fossils that have otherwise been thought related to conodonts, may represent fossilised chaetognath spines. It has been suggested that they may have been organic like modern chaetognath spines during life and only become secondarily phosphatised during fossilisation (Szaniawski 2002). The Early Cambrian Pseudoconodontida were similar in overall appearance but differed in internal microstructure: whereas protoconodonts grew by basal addition of growth laminae from a base that was presumably partially embedded in secretory tissue, pseudoconodonts have non-laminate, longitudinally fibrous structure comprised of rod-like crystallites and were probably entirely surficial (Landing 1995).

Characters (from Szaniawski 2002): Bilaterally symmetrical animals with body divided between a rounded or triangular flattened head and an elongate, fluid-filled trunk supported by a hydroskeleton. Head usually armed with one or two paired rows of small teeth located antero-ventrally and a group of large grasping spines postero-laterally; teeth and spines covered by a retractable hood when at rest. Trunk with one or two pairs of lateral fins and a terminal horizontal tail fin. Trunk and dorsal surface of head with multi-layered epidermis lacking cuticle; ventral surface of head with mono-layered epidermis and cuticle. Subterminal anus.

Chaetognatha [Chaetognathi]
    |--Protosagitta Hu in Chen et al. 2002 EL11, HE04
    |    `--P. spinosa Hu 2002 C12
    |--Paraspadella [Phragmophora] WT06
    |    `--P. gotoi WT06
    |--Protoconodontida L95
    |    |  i. s.: ‘Protohertzina’ cultrata Missarzhevsky 1977 L95
    |    |--Yunnanodus Wang & Jiang in Jiang 1980 L95
    |    |    `--Y. doleres Wang & Jiang 1980 WS93
    |    |--Amphigeisina [Amphigeisinacea, Amphigeisinidae] L95
    |    |    `--A. danica (Poulsen 1966) L95
    |    |--Gapparodus Abaimova 1978 L95
    |    |    `--G. bisulcata (Müller 1959) L95
    |    |--Hertzina Müller 1959 L95
    |    |    |--*H. americana Müller 1959 M62
    |    |    `--H. elongata Muller 1959 L95
    |    `--Phakelodus Miller 1984 S02, L95
    |         |--P. elongatus (Zhang in An et al. 1983) S02
    |         |--P. savitzkyi S02
    |         `--P. tenuis (Müller 1959) [=Prooneotodus tenuis] S02
    |--Sagitta Quoy & Gaimard 1827 [Aphragmophora] S02
    |    |--S. betodi ZS10
    |    |--S. bipunctata D56
    |    |--S. crassa KW95
    |    |--S. elegans GD00
    |    |    |--S. e. elegans CS77
    |    |    `--S. e. artica CS77
    |    |--S. enflata B26
    |    |--S. hamata [=Krohnia hamata] O01
    |    |--S. hexaptera B70
    |    |--S. lyra B70
    |    |--S. maxima Conant 1896 S02
    |    |--S. nagae ZS10
    |    |--S. scrippsae B70
    |    |--S. serratodentata B26
    |    |--S. tasmanica B70
    |    `--S. zetesios B70
    `--Pseudoconodontida L95
         |--Protohertzinacea L95
         |    |--Maldeotaia bandalica Singh & Shukla 1981 L95
         |    |--Protohertzina Missarzhevsky 1973 S02
         |    |    `--*P. anabarica Missarzhevsky 1973 [incl. P. unguliformis Missarzhevsky 1973] L95
         |    `--Mongolodus Missarzhevsky 1977 [incl. Ganloudina He in Yang & He 1984 L95
         |         `--‘Ganloudina’ symmetrica He in Yang & He 1984 L95
         `--Strictocorniculacea L95
              |--Rhombocorniculum Walliser 1958 DJ71 [Rhombocorniculidae L95]
              |    `--R. cancellatum (Cobbold 1921) L95 (see below for synonymy)
              `--Strictocorniculidae L95
                   |  i. s.: ‘Rhombocorniculum’ insolutum Missarzhevsky in Missarzhevsky & Mambetov 1981 L95
                   |--Mongolitubulus squamifer Missarzhevsky 1977 WS93
                   |--Rushtonites Hinz 1987 L95
                   |    `--R. spinosus Hinz 1987 WS93
                   `--Strictocorniculum Landing 1995 L95
                        `--*S. vanallerum Landing 1995 L95

Chaetognatha incertae sedis:
  Paucijaculum samamithion Schram 1973 S02
  Spadella [Spadellidae] HO09
    |--S. cephaloptera HO09
    `--S. nunezi Casanova & Moreau 2004 F05
  Eukrohnia hamata B26
  Pterosagitta draco B26
  Flaccisagitta enflata HO09
  Eognathacantha Chen & Huang 2002 CH02
    `--*E. ercainella Chen & Huang 2002 CH02
  Coelocerodontus Ethington 1959 S02
    |--*C. trigonius Ethington 1959 DJ71
    |--C. biconvexus Bultynck 1970 WSQ86
    |--C. burkei Druce & Jones 1971 DJ71
    |--C. primitivus (Müller 1959) [=Furnishina primitiva] DJ71
    |--C. rotundatus Druce & Jones 1971 DJ71
    `--C. tricarinatus (Nogami 1967) [=Hertzina tricarinata] DJ71
  Aidanosagitta KS03
  Titerina rokycanensis Kraft & Mergl 1989 HE04
  Pseudosagitta maxima HE04

Rhombocorniculum cancellatum (Cobbold 1921) L95 [=Helenia cancellata DJ71; incl. *R. comleyense Walliser 1958 DJ71]

*Type species of generic name indicated


[B26] Bigelow, H. B. 1926. Plankton of the offshore waters of the Gulf of Maine. Bulletin of the Bureau of Fisheries 40 (2): 1–509.

[B70] Briggs, J. C. 1970. A faunal history of the North Atlantic Ocean. Systematic Zoology 19 (1): 19–34.

[C12] Chen, J.-Y. 2012. Evolutionary scenario of the early history of the animal kingdom: evidence from Precambrian (Ediacaran) Weng’an and Early Cambrian Maotianshan biotas, China. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 239–379. Springer.

[CH02] Chen, J.-Y., & D.-Y. Huang. 2002. A possible Lower Cambrian chaetognath. Science 298: 187.

[CS77] Cramp, S., & K. E. L. Simmons (eds) 1977. Handbook of the Birds of Europe, the Middle East and North Africa: The Birds of the Western Palaearctic vol. 1. Ostrich to Ducks. Oxford University Press: Oxford.

[D56] Dawes, B. 1956. The Trematoda with special reference to British and other European forms. University Press: Cambridge.

[DJ71] Druce, E. C., & P. J. Jones. 1971. Cambro-Ordovician conodonts from the Burke River Structural Belt, Queensland. Commonwealth of Australia, Bureau of National Development, Bureau of Mineral Resources, Geology and Geophysics, Bulletin 110: 1–159.

[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.

[F05] Fernández, J. 2005. Noticia de nuevos táxones para la ciencia en el ámbito Íbero-Balear y Macaronésico. Nuevos táxones animales descritos en la península Ibérica y Macaronesia desde 1994 (IX). Graellsia 61 (2): 261–282.

[GD00] Giribet, G., D. L. Distel, M. Polz, W. Sterrer & W. C. Wheeler. 2000. Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology. Systematic Biology 49: 539–562.

[HO09] Hejnol, A., M. Obst, A. Stamatakis, M. Ott, G. W. Rouse, G. D. Edgecombe, P. Martinez, J. Baguña, X. Bailly, U. Jondelius, M. Wiens, W. E. G. Müller, E. Seaver, W. C. Wheeler, M. Q. Martindale, G. Giribet & C. W. Dunn. 2009. Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings of the Royal Society of London Series B—Biological Sciences 276 (1677): 4261–4270.

[HE04] Hints, O., M. Eriksson, A. E. S. Högström, P. Kraft & O. Lehnert. 2004. Worms, wormlike and sclerite-bearing taxa. In: Webby, B. D., F. Paris, M. L. Droser & I. G. Percival (eds) The Great Ordovician Biodiversification Event pp. 223–230. Columbia University Press.

[KS03] Kasatkina, A. P., & E. N. Selivanova. 2003. Composition of the genus Aidanosagitta (Chaetognatha), with descriptions of new species from shallow bays of the northwestern Sea of Japan. Russian Journal of Marine Biology 29 (5): 296–304.

[KW95] Katayama, T., H. Wada, H. Furuya, N. Satoh & M. Yamamoto. 1995. Phylogenetic position of the dicyemid Mesozoa inferred from 18S rDNA sequences. Biological Bulletin 189: 81–90.

[L95] Landing, E. 1995. Upper Placentian–Branchian series of mainland Nova Scotia (middle–upper Lower Cambrian): faunas, paleoenvironments, and stratigraphic revision. Journal of Paleontology 69 (3): 475–495.

[M62] Müller, K. J. 1962. Supplement to systematics of conodonts. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W246–W249. Geological Society of America, and University of Kansas Press.

[O01] Ohlin, A. 1901. On a new "bipolar" schizopod. Annals and Magazine of Natural History, series 7, 7: 371–374.

[S02] Szaniawski, H. 2002. New evidence for the protoconodont origin of chaetognaths. Acta Paleontologica Polonica 47 (3): 405–419.

[WT06] Waeschenbach, A., M. J. Telford, J. S. Porter & D. T. J. Littlewood. 2006. The complete mitochondrial genome of Flustrellidra hispida and the phylogenetic position of Bryozoa among the Metazoa. Molecular Phylogenetics and Evolution 40 (1): 195–207.

[WSQ86] Wang C.-Y., Shi C.-G. & Qu G.-S. 1986. Conodonts and ostracodes from the Devonian “Heitai Formation” of Mishan County, Heilongjiang Province. Acta Micropalaeontologica Sinica 3 (2): 205–214.

[WS93] Wills, M. A., & J. J. Sepkoski Jr. 1993. Problematica. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 543–554. Chapman & Hall: London.

[ZS10] Zhang, G.-T., S. Sun, Z.-L. Xu & Q.-L. Zhang. 2010. Unexpected dominance of the subtropical copepod Temora turbinata in the temperate Changjiang River estuary and its possible causes. In: China-Russia Bilateral Symposium: Proceedings of the China-Russia Bilateral Symposium of "Comparison on Marine Biodiversity in the Northwest Pacific Ocean", 10–11 October 2010, Qingdao (China) pp. 100–102. Institute of Oceanology, Chinese Academy of Sciences; A. V. Zhirmunsky Institute of Marine Biology, Far East Branch of the Russian Academy of Sciences.

Last updated: 25 December 2021.

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