phylum of marine worms

Chaetognatha,[1] commonly known as arrow worms, are a phylum of small predatory marine animals. They are present in huge numbers in plankton worldwide.

Arrow worms
Temporal range: Lower Cambrian to Recent
Spadella cephaloptera
Scientific classification

Leuckart 1854

About 20% of the known species are benthic and can attach to algae or 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 (0.079 to 4.724 in).

Despite the huge numbers, there are only about 120 modern species in 20 genera.[2] Some species are known to use the neurotoxin tetrodotoxin to subdue their prey.[3]

Chaetognaths appear to have originated in the Cambrian Period. Complete body fossils have been described from China,[4][5] Greenland [6] and the Middle Cambrian Burgess Shale of British Columbia.[7]

These animals are usually considered a type of protostome, closely related to rotifers.


  1. meaning hair-jaws
  2. Bone Q.; et al. (1991). The biology of chaetognaths. London: Oxford University Press.
  3. Thuesen, E.V.; et al. (1991), "The tetrodotoxin venom of chaetognaths", in Bone, Q. (ed.), The biology of chaetognaths, London: Oxford University Press, pp. 55–60
  4. Chen J.-Y., and D.-Y. Huang. 2002. A possible Lower Cambrian chaetognath (arrow worm). Science 298:187.
  5. Hu S.-X. 2005. Taphonomy and palaeoecology of the Early Cambrian Chengjiang Biota from Eastern Yunnan, China. Berliner Paläobiologische Abhandlungen 7:1–197.
  6. Park, Tae-Yoon S.; Nielsen, Morten Lunde; Parry, Luke A.; Sørensen, Martin Vinther; Lee, Mirinae; Kihm, Ji-Hoon; Ahn, Inhye; Park, Changkun; De Vivo, Giacinto; Smith, M. Paul; Harper, David A. T.; Nielsen, Arne T.; Vinther, Jakob (2024), "A giant stem-group chaetognath", Science Advances, 10, doi:10.1126/sciadv.adi6678, PMC 10796117
  7. Szaniawski H. 2005. Cambrian chaetognaths recognized in Burgess Shale fossils. Acta Palaeontologica Polonica 50:1-8.