View source for Chaetognatha

===Locomotion===
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the [[fin ray]]s of fish, with which they are not [[homology (biology)|homologous]]. Unlike those of vertebrates, these lateral fins are composed of a thickened [[basement membrane]] extending from the [[epidermis (zoology)|epidermis]]. An additional caudal fin covers the post-anal tail.<ref name=IZ/> Two chaetognath species, ''[[Caecosagitta macrocephala]]'' and ''[[Eukrohnia fowleri]]'', have [[bioluminescent]] organs on their fins.<ref>{{cite journal| vauthors = Haddock SH, Case JF |date=20 January 1994 |title=A bioluminescent chaetognath |journal=[[Nature (journal)|Nature]] |volume=367 |issue=6460 |pages=225–226 |doi=10.1038/367225a0|bibcode=1994Natur.367..225H |s2cid=4362422 }}</ref><ref>{{cite journal | vauthors = Thuesen EV, Goetz FE, Haddock SH | title = Bioluminescent organs of two deep-sea arrow worms, Eukrohnia fowleri and Caecosagitta macrocephala, with further observations on Bioluminescence in chaetognaths | journal = The Biological Bulletin | volume = 219 | issue = 2 | pages = 100–11 | date = October 2010 | pmid = 20972255 | doi = 10.1086/BBLv219n2p100  | s2cid = 14502203 | author3-link = Steven Haddock }}</ref>

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.<ref>{{cite journal| vauthors = Jordan CE |year=1992 |title=A model of rapid-start swimming at intermediate Reynolds number: undulatory locomotion in the chaetognath ''Sagitta elegans'' |journal=[[Journal of Experimental Biology]] |volume=163 |issue=1 |pages=119–137 |doi=10.1242/jeb.163.1.119 |url=http://jeb.biologists.org/content/163/1/119|doi-access=free }}</ref> To avoid sinking they need to swim regularly, but many species have [[ammonium]]-filled vacuolated cells in the trunk, which gives them close to neutral buoyancy.<ref>[https://www.google.no/books/edition/Marine_Plankton/NuUWDgAAQBAJ?hl=en&gbpv=1&dq=Chaetognatha+vacuole+sacs&pg=PA552&printsec=frontcover Marine PLankton: A Practical Guide to Ecology, Methodology, and Taxonomy]</ref><ref>[https://www.google.no/books/edition/The_Light_and_Smith_Manual/64jgZ1CfmB8C?hl=en&gbpv=1&dq=Sagittidae+ammonia-filled&pg=PA907&printsec=frontcover The Light & Smith Manual: Intertidal invertebrates from Central California to Oregon]</ref> Muscle movements have been described as among the fastest of any animals.<ref name="Perez2021"/> Muscles are directly excitable by electrical currents or strong [[potassium|K+]] solutions; the main neuromuscular transmitter is acetylcholine.<ref name="Perez2021"/>