What is Chaetognatha?
{{Taxobox | image =Chaetoblack.png | image_width = 120px | image_caption = ''Spadella cephaloptera'' | name = Arrow worms | fossil_range = <small>Lower Cambrian to Recent</small> | regnum = [[Animal]]ia | superphylum = [[Platyzoa]] | phylum = '''Chaetognatha''' | phylum_authority = Leuckart 1854 | subdivision_ranks = Classes | subdivision = }} '''Chaetognatha''',<ref>meaning ''hair-jaws''</ref> commonly known as '''arrow worms''', are a [[Phylum (biology)|phylum]] of small [[predator]]y [[Sea|marine]] animals. They are present in huge numbers in [[plankton]] worldwide. About 20% of the known species are [[benthic]] and can attach to [[algae]] or [[rock (geology)|rock]]s. They are found in all marine waters from surface tropical waters and shallow [[tide pool]]s to the deep sea and [[polar region]]s. Most chaetognaths are [[transparent]] and are torpedo shaped, but some deep-sea species are orange. They range in size from {{convert|2|to|120|mm|in}}. Despite the huge numbers, there are only about 120 modern [[species]] in 20 [[genera]].<ref> {{cite book | author = Bone Q. | display-authors = etal | year = 1991 | title = The biology of chaetognaths | publisher = Oxford University Press | place = London }} </ref> Some species are known to use the [[neurotoxin]] ''tetrodotoxin'' to subdue their [[prey]].<ref> {{Citation | last = Thuesen | first = E.V. | contribution = The tetrodotoxin venom of chaetognaths | editor-last = Bone | editor-first = Q. | display-authors = etal | title = The biology of chaetognaths | pages = 55–60 | publisher = Oxford University Press | place = London | year = 1991}}</ref> Chaetognaths appear to have originated in the [[Cambrian]] Period. Complete body fossils have been described from [[China]],<ref>Chen J.-Y., and D.-Y. Huang. 2002. A possible Lower Cambrian chaetognath (arrow worm). ''Science'' '''298''':187.</ref><ref>Hu S.-X. 2005. Taphonomy and palaeoecology of the Early Cambrian Chengjiang Biota from Eastern Yunnan, China. ''Berliner Paläobiologische Abhandlungen