Chaetognatha (meaning bristle-jaws) is a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and can attach to algae and rocks. They are found in all marine waters, from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from 2 to 120 millimetres (0.1 to 4.7 in).

There are more than 120 modern species assigned to over 20 genera.[3] Despite the limited diversity of species, the number of individuals is large.[4]

Arrow worms are usually considered a type of protostome that do not belong to either Ecdysozoa or Lophotrochozoa.

Arrow worms
Temporal range: Lower Cambrian–Recent[1]
Spadella cephaloptera
Scientific classification
Kingdom: Animalia
(unranked): Spiralia
Clade: Gnathifera
Leuckart, 1854
Class: Sagittoidea
Claus & Grobben, 1905 [2]


MEB back
The jaw organ of a Chaetognath of the genus Sagitta

Chaetognaths are transparent or translucent dart-shaped animals covered by a cuticle. The body is divided into a distinct head, trunk, and tail. There are between four and fourteen hooked, grasping spines on each side of their head, flanking a hollow vestibule containing the mouth. The spines are used in hunting, and covered with a flexible hood arising from the neck region when the animal is swimming. All chaetognaths are carnivorous, preying on other planktonic animals.[5]

The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the fin rays of fish, with which they are not homologous, however: unlike those of vertebrates, these are composed of a thickened basement membrane extending from the epidermis. An additional caudal fin covers the post-anal tail.[5] Two chaetognath species, Caecosagitta macrocephala and Eukrohnia fowleri, have bioluminescent organs on their fins.[6][7]

Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins with stabilization and steering.[8] Some species are known to use the neurotoxin tetrodotoxin to subdue prey.[9]

The body cavity is lined by peritoneum, and therefore represents a true coelom, and is divided into one compartment on each side of the trunk, and additional compartments inside the head and tail, all separated completely by septa. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no respiratory or circulatory systems.

The mouth opens into a muscular pharynx, which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to an anus just forward of the tail. The intestine is the primary site of digestion and includes a pair of diverticula near the anterior end.[5] Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin and anus.

The nervous system is reasonably simple, consisting of a ganglionated nerve ring surrounding the pharynx. The dorsal ganglion is the largest, but nerves extend from all the ganglia along the length of the body. Chaetognaths have two compound eyes, each consisting of a number of pigment-cup ocelli fused together. In addition, there are a number of sensory bristles arranged in rows along the side of the body, where they probably perform a function similar to that of the lateral line in fish. An additional, curved, band of sensory bristles lies over the head and neck.[5]

The arrow worm rhabdomeres are derived from microtubules 20 nm long and 50 nm wide, which in turn form conical bodies that contain granules and thread structures. The cone body is derived from a cilium.[10]


All species are hermaphroditic, carrying both eggs and sperm.[4] Each animal possesses a pair of testes within the tail, and a pair of ovaries in the posterior region of the main body cavity. Immature sperm are released from the testes to mature inside the cavity of the tail, and then swim through a short duct to a seminal vesicle where they are packaged into a spermatophore.[5](Sexually and Asexually)

During mating, each individual places a spermatophore onto the neck of its partner after rupture of the seminal vesicle. The sperm rapidly escape from the spermatophore and swim along the midline of the animal until they reach a pair of small pores just in front of the tail. These pores connect to the oviducts, into which the developed eggs have already passed from the ovaries, and it is here that fertilisation takes place.[5]

The eggs are planktonic, or attached to algae, and hatch into miniature versions of the adult, without a well-defined larval stage.[5]


Chaetognaths are traditionally classed as deuterostomes by embryologists. Lynn Margulis and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification.[11] Molecular phylogenists, however, consider them to be protostomes. Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification.[12] The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basal ecdysozoan or lophotrochozoan.[13] Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny.[14] This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they might have retained deuterostome characters inherited from early bilaterian ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.[15] Studies of arrow worms' nervous systems suggests they should be placed within the protostomes.[16][17] According to 2017 and 2019 papers, chaetognaths appear related to gnathiferans.[18][19]

Fossil record

Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described.[1] Chaetognaths appear to have originated in the Cambrian Period. Complete body fossils have been formally described from the Lower Cambrian Maotianshan shales of Yunnan, China (Eognathacantha ercainella Chen & Huang[20] and Protosagitta spinosa Hu[21]) and the Middle Cambrian Burgess Shale of British Columbia (Oesia disjuncta Walcott[22]), a view challenged by Conway Morris (2009). A more recent chaetognath, Paucijaculum samamithion Schram, has been described from the Mazon Creek biota from the Pennsylvanian of Illinois. Chaetognaths were thought possibly to be related to some of the animals grouped with the conodonts. The conodonts themselves, however, are thought to be related to the vertebrates. It is now thought that protoconodont elements (e.g., Protohertzina anabarica Missarzhevsky, 1973), are probably grasping spines of chaetognaths rather than teeth of conodonts. Previously chaetognaths in the Early Cambrian were only suspected from these protoconodont elements, but the more recent discoveries of body fossils have confirmed their presence then.[23]

Infection by giant viruses

Comparison of the size of giant viruses to a common virus (HIV) and bacteria (E. coli)
Comparison of size between various viruses and the bacteria E. coli

In 2018, reanalysis of electron microscopy photographs from the 1980s allowed scientists to identify a giant virus (Meelsvirus) infecting Adhesisagitta hispida; its site of multiplication is nuclear and the virions (length: 1.25 μm) are enveloped.[24] In 2019, reanalysis of other previous studies has shown that structures that were taken in 1967 for bristles present on the surface of the species Spadella cephaloptera,[25] and in 2003, for bacteria infecting Paraspadella gotoi,[26] were in fact enveloped and spindle-shaped giant viruses with a cytoplasmic site of multiplication. The viral species infecting P. gotoi, whose maximum length is 3.1 μm, has been named Klothovirus casanovai (Klotho being the Greek name for one of the three Fates whose attribute was a spindle, and casanovai, in tribute to Pr J.-P. Casanova who devoted a large part of his scientific life to the study of chaetognaths). The other species has been named Megaklothovirus horridgei (in tribute to the first author of the 1967 article). On a photograph, one of the viruses M. horridgei, although truncated, is 3.9 μm long, corresponding to about twice the length of the bacteria Escherichia coli. Many ribosomes are present in virions but their origin remains unknown (cellular, viral or only partly viral). Moreover, it must be noted that, to date, giant viruses known to infect metazoans are exceptionally rare.


  1. ^ a b Vannier J, Steiner M, Renvoisé E, Hu SX, Casanova JP (March 2007). "Early Cambrian origin of modern food webs: evidence from predator arrow worms". Proceedings of the Royal Society B: Biological Sciences. 274 (1610): 627–33. doi:10.1098/rspb.2006.3761. PMC 2197202. PMID 17254986.
  2. ^ "Sagittoidea Claus and Grobben, 1905". Integrated Taxonomic Information System. Retrieved February 8, 2012.
  3. ^ "World Register of Marine Species".
  4. ^ a b Bone Q, Kapp H, Pierrot-Bults AC, eds. (1991). The Biology of Chaetognaths. London: Oxford University Press. ISBN 978-0-19-857715-7.
  5. ^ a b c d e f g Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 1046–1050. ISBN 978-0-03-056747-6.
  6. ^ Haddock SH, Case JF (20 January 1994). "A bioluminescent chaetognath". Nature. 367 (6460): 225–226. Bibcode:1994Natur.367..225H. doi:10.1038/367225a0.
  7. ^ Thuesen EV, Goetz FE & Haddock SH (October 2010). "Bioluminescent organs of two deep-sea arrow worms, Eukrohnia fowleri and Caecosagitta macrocephala, with further observations on Bioluminescence in chaetognaths". The Biological Bulletin. 219 (2): 100–11. doi:10.1086/BBLv219n2p100. PMID 20972255.
  8. ^ Jordan CE (1992). "A model of rapid-start swimming at intermediate Reynolds number: undulatory locomotion in the chaetognath Sagitta elegans". Journal of Experimental Biology. 163 (1): 119–137.
  9. ^ Bone Q, Kapp H, Pierrot-Bults AC, eds. (1991). "The Tetrodotoxin Venom of Chaetognaths". The Biology of chaetognaths. Oxford University Press. pp. 55–60. ISBN 978-0-19-857715-7.
  10. ^ "Photoreception". Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD . 2009.
  11. ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Margulis and Schwartz Archived November 27, 2005, at the Wayback Machine (select Margulis & Schwartz in 'Classification by')—last retrieved November 25, 2006
  12. ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Cavalier-Smith Archived November 27, 2005, at the Wayback Machine (select Cavalier-Smith in 'Classification by')—last retrieved November 25, 2006
  13. ^ Matus DQ, Copley RR, Dunn CW, Hejnol A, Eccleston H, Halanych KM, Martindale MQ, Telford MJ (August 2006). "Broad taxon and gene sampling indicate that chaetognaths are protostomes". Current Biology. 16 (15): R575–6. doi:10.1016/j.cub.2006.07.017. PMID 16890509.
  14. ^ Marlétaz F, Martin E, Perez Y, Papillon D, Caubit X, Lowe CJ, Freeman B, Fasano L, Dossat C, Wincker P, Weissenbach J, Le Parco Y (August 2006). "Chaetognath phylogenomics: a protostome with deuterostome-like development". Current Biology. 16 (15): R577–8. doi:10.1016/j.cub.2006.07.016. PMID 16890510.
  15. ^ Papillon D, Perez Y, Caubit X, Le Parco Y (November 2004). "Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome". Molecular Biology and Evolution. 21 (11): 2122–9. doi:10.1093/molbev/msh229. PMID 15306659.
  16. ^ Rieger V, Perez Y, Müller CH, Lipke E, Sombke A, Hansson BS, Harzsch S (February 2010). "Immunohistochemical analysis and 3D reconstruction of the cephalic nervous system in Chaetognatha: Insights into the evolution of an early bilaterian brain?". Invertebrate Biology. 129 (1): 77–104. doi:10.1111/j.1744-7410.2010.00189.x.
  17. ^ Harzsch S, Müller CH (May 2007). "A new look at the ventral nerve centre of Sagitta: implications for the phylogenetic position of Chaetognatha (arrow worms) and the evolution of the bilaterian nervous system". Frontiers in Zoology. 4: 14. doi:10.1186/1742-9994-4-14. PMC 1885248. PMID 17511857.
  18. ^ Fröbius AC, Funch P (April 2017). "Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans". Nature Communications. 8 (1): 9. Bibcode:2017NatCo...8....9F. doi:10.1038/s41467-017-00020-w. PMC 5431905. PMID 28377584.
  19. ^ Marlétaz F, Peijnenburg KT, Goto T, Satoh N, Rokhsar DS (January 2019). "A New Spiralian Phylogeny Places the Enigmatic Arrow Worms among Gnathiferans". Current Biology. 29 (2): 312–318.e3. doi:10.1016/j.cub.2018.11.042. PMID 30639106.
  20. ^ Chen JY, Huang DY (October 2002). "A possible Lower Cambrian chaetognath (arrow worm)". Science. 298 (5591): 187. doi:10.1126/science.1075059. PMID 12364798.
  21. ^ Hu SX (2005). "Taphonomy and palaeoecology of the Early Cambrian Chengjiang Biota from Eastern Yunnan, China". Berliner Paläobiologische Abhandlungen. 7: 1–197.
  22. ^ Szaniawski H (2005). "Cambrian chaetognaths recognized in Burgess Shale fossils" (PDF). Acta Palaeontologica Polonica. 50 (1): 1–8.
  23. ^ Szaniawski H (2002). "New evidence for the protoconodont origin of chaetognaths" (PDF). Acta Palaeontologica Polonica. 47 (3): 405–419.
  24. ^ Shinn GL, Bullard BL (2018-09-19). San Martin C (ed.). "Ultrastructure of Meelsvirus: A nuclear virus of arrow worms (phylum Chaetognatha) producing giant "tailed" virions". PLOS ONE. 13 (9): e0203282. Bibcode:2018PLoSO..1303282S. doi:10.1371/journal.pone.0203282. PMC 6145532. PMID 30231047.
  25. ^ Horridge GA, Boulton PS (1967-11-14). "Prey detection by Chaetognatha via a vibration sense". Proceedings of the Royal Society of London. Series B. Biological Sciences. 168 (1013): 413–419. Bibcode:1967RSPSB.168..413H. doi:10.1098/rspb.1967.0072.
  26. ^ Casanova JP, Duvert M, Goto T (October 2003). "Ultrastructural study and ontogenesis of the appendages and related musculature of Paraspadella (Chaetognatha)". Tissue & Cell. 35 (5): 339–51. doi:10.1016/S0040-8166(03)00055-7. PMID 14517101.

External links


Aphragmophora is an order of sagittodieans in the phylum Chaetognatha.

Archeterokrohnia docrickettsae

Archeterokrohnia docrickettsae is a type of deep-sea marine arrow worm. It is the largest species in the Archeterokrohnia genus and the first to be found alive. Additionally this species displays an unusual color pattern of orange on its head and trunk with a translucent tail.


The bilateria , bilaterians, or triploblasts, are animals with bilateral symmetry, i.e., they have a head (anterior) and a tail (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.The bilateria are a major group of animals, including the majority of phyla but not sponges, ctenophores, placozoans, and cnidarians. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Nearly all are bilaterally symmetrical, or approximately so; the most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others display primary body cavities (deriving from the blastocoel, as pseudocoeloms) or secondary cavities (that appear de novo, for example the coelom).


Deuterostomes (taxonomic term: Deuterostomia; meaning "second mouth" in Greek) constitute a superphylum of animals. It is a sister clade of Protostomia, with which it forms the Nephrozoa clade.

Deuterostomia is a subtaxon of the Bilateria branch of the subkingdom Eumetazoa, within Animalia, and are distinguished from protostomes by their deuterostomic embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes, it becomes the mouth. (There are some occurrences of deuterostomy among protostomes.)Deuterostomes are also known as enterocoelomates because their coelom develops through enterocoely.

Many groups of organisms originally thought to have belonged to this group (e.g. Lophophorata, Chaetognatha) have been placed elsewhere, to the point where the possibility of the term deuterostome being deprecated is considered.An undisputed extant group in the deuterostome clade is the Chordata, i.e. the vertebrates and their kin.

The other possible deuterostome group is the Ambulacraria: Echinodermata (starfish, sea urchins, sea cucumbers) + Hemichordata (acorn worms and graptolites).

In 2019, there was cautious support for the assessment that the Ambulacraria are sister to the Xenacoelomorpha together forming the Xenambulacraria, probably as basal Bilateria or deuterostome. Without Ambulacraria, Deuterostomes would then be a junior synonym to the Chordata.


Eukrohniidae is a family of sagittoideans in the order Phragmophora. It consists of a single genus, Eukrohnia von Ritter-Záhony, 1909.

Gelatinous zooplankton

Gelatinous zooplankton are fragile animals that live in the water column in the ocean. They have very delicate bodies that are easily damaged or destroyed. Gelatinous zooplankton are often transparent. All jellyfish are gelatinous zooplankton, but not all gelatinous zooplankton are jellyfish. The most commonly encountered organisms include ctenophores, medusae, salps, and Chaetognatha in coastal waters. However, almost all marine phyla, including Annelida, Mollusca and Arthropoda, contain gelatinous species, but many of those odd species live in the open ocean and the deep sea and are less available to the casual ocean observer. Gelatinous zooplankton have also been called "Gelata".

Gnathifera (clade)

Gnathifera (from the Greek gnáthos, “jaw”, and the Latin -fera, “bearing”) is a clade of generally small spiralians characterized by complex jaws made of chitin. It comprises the phyla Gnathostomulida, Rotifera, Micrognathozoa, and Chaetognatha. It may also include the Cycliophora.Gnathiferans include some of the most abundant phyla. Rotifers are among the most diverse and abundant freshwater animals and chaetognaths are among the most abundant marine plankton.


Heterokrohniidae is a family of sagittoideans in the order Phragmophora.

List of Chengjiang Biota species by phylum

List of Chengjiang Biota species is of fossils found at Maotianshan Shales whose most famous assemblage of organisms are referred to as the Chengjiang biota.

Louis Joubin

Louis Marie Adolphe Olivier Édouard Joubin (27 January 1861 in Épinal – 24 April 1935) was a professor at the Muséum national d'Histoire naturelle in Paris. He published works on nemerteans, chaetognatha, cephalopods, and other molluscs.He served as an assistant to Henri de Lacaze-Duthiers, subsequently becoming director of the laboratories at Banyuls-sur-Mer (1882) and Roscoff (1884). Later on, he became an instructor at the University of Rennes, and in 1903 succeeded Edmond Perrier as chaire des mollusques, des vers et des zoophytes at the Muséum national d'Histoire naturelle (from 1917 onward his title was chaire des mollusques). In 1906 he was chosen by Albert I, Prince of Monaco to be in charge of instruction at the Institut océanographique.In 1905 he was named president of the Société zoologique de France. In 1920 he became a member of the Académie des Sciences.Joubin's squid (Joubiniteuthis portieri) is named for him, as is Scolymastra joubini, a hexactinellid sponge whose lifespan is purportedly 10,000 years.

Marine worm

Any worm that lives in a marine environment is considered a marine worm. Marine worms are found in several different phyla, including the Platyhelminthes, Nematoda, Annelida (segmented worms), Chaetognatha, Hemichordata, and Phoronida. For a list of marine animals that have been called "sea worms", see sea worm.

Many of these worms have specialized tentacles used for exchanging oxygen and carbon dioxide and also may be used for reproduction.

Some marine worms are tube worms, of which the giant tube worm lives in waters near underwater volcanoes, and can withstand temperatures up to 90 degrees Celsius or about 194 degrees Fahrenheit.

Some worms can live in the trench. These worms were first discovered in the Pacific Ocean off the Galápagos Islands.

In recent years, marine worms (especially those found in the ocean) have been observed ingesting microplastic particles found in the oceans. This trend is concerning many scientists, as marine worms act as an important food source for many fish and wading birds. Marine Worms play as Keystone Species in an ecosystem, and the introduction of plastic in the oceans will not only diminish the growth rates of the marine worms, but also affect the food chain of that ecosystem.


Phragmophora is an order of sagittoideans in the phylum Chaetognatha.


Protostomia (from Greek πρωτο- proto- "first" and στόμα stoma "mouth") is a clade of animals. Together with the deuterostomes and xenacoelomorpha, its members make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development and is based on the embryological origins of the mouth and anus.

In most, but not all protostomes, the mouth forms first, then the anus, whereas the reverse is true in deuterostomes.


Pterokrohniidae is a family of sagittoideans in the order Aphragmophora. I consists of a single genus, Pterokrohnia Srinivasan, 1986, which consists of a single species, Pterokrohnia arabica Srinavasan, 1986.


Pterosagittidae is a family of sagittoideans in the order Aphragmophora. It consists of a single genus, Pterosagitta Costa, 1869, which consists of a single species, Pterosagitta draco (Krohn, 1853).

Sagitta (arrowworm)

Sagitta is a genus of Chaetognatha, a phylum commonly known as arrowworms or arrow worms. Phylum Chaetognatha comprises small marine worms.

This genus is characterized in part by the distribution of the cilia on the body, the thick rays in the fins, and hooks which are not serrated.As of 2007 there are 15 species. More have since been described.Species include:

Sagitta abyssicola

Sagitta bedoti

Sagitta bipunctata – Commons

Sagitta bruuni

Sagitta euneritica

Sagitta euxina

Sagitta glacialis

S. g. baltica

S. g. glacialis

Sagitta izuensis

Sagitta kussakini

Sagitta modesta

Sagitta nagae

Sagitta nutana

Sagitta pulchra

Sagitta sceptrum

Sagitta setosa

Sagitta sublica


Sagittidae is a family of sagittoideans in the order Aphragmophora.


Spadellidae is a family of sagittoideans in the order Phragmophora. Spadellidae prey on plankton and commonly reside in the epipelagic zone of the ocean.


Worms are many different distantly related animals that typically have a long cylindrical tube-like body and no limbs. Worms vary in size from microscopic to over 1 metre (3.3 ft) in length for marine polychaete worms (bristle worms), 6.7 metres (22 ft) for the African giant earthworm, Microchaetus rappi, and 58 metres (190 ft) for the marine nemertean worm (bootlace worm), Lineus longissimus. Various types of worm occupy a small variety of parasitic niches, living inside the bodies of other animals. Free-living worm species do not live on land, but instead, live in marine or freshwater environments, or underground by burrowing.

In biology, "worm" refers to an obsolete taxon, vermes, used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be paraphyletic. The name stems from the Old English word wyrm. Most animals called "worms" are invertebrates, but the term is also used for the amphibian caecilians and the slowworm Anguis, a legless burrowing lizard. Invertebrate animals commonly called "worms" include annelids (earthworms and marine polychaete or bristle worms), nematodes (roundworms), platyhelminthes (flatworms), marine nemertean worms ("bootlace worms"), marine Chaetognatha (arrow worms), priapulid worms, and insect larvae such as grubs and maggots.

Worms may also be called helminths, particularly in medical terminology when referring to parasitic worms, especially the Nematoda (roundworms) and Cestoda (tapeworms) which reside in the intestines of their host. When an animal or human is said to "have worms", it means that it is infested with parasitic worms, typically roundworms or tapeworms. Lungworm is also a common parasitic worm found in various animal species such as fish and cats.

About plankton
By size
Related topics
Extant Animal phyla
Extant life phyla/divisions by domain


This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.