Editing Chaetognatha (section)

==Anatomy==
[[File:Ten species of chaetognaths.webp|thumb|right|400px|Drawing of ten different chaetognath species, showing morphological similarity and diversity.]]
Chaetognaths are transparent or translucent dart-shaped animals covered by a [[cuticle]]. They range in length between 1.5 mm to 105 mm in the Antarctic species ''[[Pseudosagitta gazellae]]''.<ref name="Pauly2021"/> Body size, either between individuals in the same species or between different species, seems to increase with decreasing temperature.<ref name="Pauly2021"/> The body is divided into a distinct head, trunk, and tail. About 80% of the body is occupied by primary longitudinal [[muscle]]s.<ref name="Perez2021"/>  

===Head and digestive system===
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. Spines and teeth are made of α-[[chitin]], and the head is protected by a chitinous armature.<ref name="Perez2021"/>  
[[File:MEB back.png|thumb|left|180px|The jaw organ of a chaetognath of the genus ''[[Sagitta (arrowworm)|Sagitta]]'']]
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 [[diverticulum|diverticula]] near the anterior end.<ref name=IZ/> Materials are moved about the body cavity by [[cilia]]. Waste materials are simply excreted through the skin and anus. Eukrohniid species possess an oil vacuole closely associated with the gut. This organ contains wax esters which may assist reproduction and growth outside of the production season for ''Eukrohnia hamata'' in Arctic seas.<ref>{{Cite journal|last1=Grigor|first1=Jordan J.|last2=Schmid|first2=Moritz S.|last3=Fortier|first3=Louis|date=2017-11-01|title=Growth and reproduction of the chaetognaths Eukrohnia hamata and Parasagitta elegans in the Canadian Arctic Ocean: capital breeding versus income breeding|url=https://academic.oup.com/plankt/article/39/6/910/4344873|journal=Journal of Plankton Research|language=en|volume=39|issue=6|pages=910–929|doi=10.1093/plankt/fbx045|issn=0142-7873|doi-access=free}}</ref> Owing to the position of the oil vacuole in the center of the tractus, the organ may also have implications for buoyancy, trim and locomotion.<ref>{{Cite journal|last=Pond|first=David W.|date=2012-06-01|title=The physical properties of lipids and their role in controlling the distribution of zooplankton in the oceans|url=https://academic.oup.com/plankt/article/34/6/443/1576677|journal=Journal of Plankton Research|language=en|volume=34|issue=6|pages=443–453|doi=10.1093/plankt/fbs027|issn=0142-7873|doi-access=free}}</ref>

Usually chaetognaths are not pigmented, however the intestines of some deep-sea species contain orange-red carotenoid pigments.<ref name="Perez2021"/>  

[[File:Chaetomorpho.png|thumb|right|160px|Anatomy of a chaetognath (''Spadella cephaloptera'')]]

===Nervous and sensory systems===
The nervous system is reasonably simple and shows a typical [[protostome]] anatomy,<ref name="Perez2021"/> consisting of a [[ganglion]]ated nerve ring surrounding the pharynx. The brain is composed of two distinct functional domains: the anterior neuropil domain and the posterior neuropil domain. The former probably controls head muscles moving the spines and the digestive system. The latter is linked to eyes and the corona ciliata. A putative sensory structure of unknown function, the retrocerebral organ, is also hosted by the posterior neuropil domain.<ref name="Perez2021"/> The ventral 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; some deep-sea and [[troglobitic]] species have unpigmented or absent eyes.<ref name="Perez2021"/> 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.<ref name=IZ/> Almost all chaetognaths have "indirect" or "inverted" eyes, according to the orientation of [[Photoreceptor cell|photoreceptor]] cells; only some [[Eukhroniidae]] species have "direct" or "everted" eyes.<ref name="Perez2021"/> A unique feature of the chaetognath eye is the lamellar structure of photoreceptor membranes, containing a grid of 35–55 nm wide circular pores.<ref name="Perez2021"/>

A significant mechanosensory system, composed of ciliary receptor organs, detects vibrations, allowing chaetognaths to detect the swimming motion of potential prey. Another organ on the dorsal part of the neck, the corona ciliata, is probably involved in chemoreception.<ref name="Perez2021"/>

===Internal organs===
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 excretory or respiratory systems.<ref>[https://books.google.com/books?id=9IWaqAOGyt4C&dq=Chaetognatha+circulatory+excretory+respiratory+systems&pg=PA343 Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth]</ref><ref name="Perez2021"/> While often said to lack a circulatory system, chaetognaths do have a rudimentary hemal system resembling those of [[Annelida|annelids]].<ref name="Perez2021"/>

The arrow worm [[rhabdomere]]s are derived from [[microtubule]]s 20&nbsp;nm long and 50&nbsp;nm wide, which in turn form conical bodies that contain granules and thread structures. The cone body is derived from a [[cilium]].<ref>"Photoreception". Encyclopædia Britannica from [[Encyclopædia Britannica 2006 Ultimate Reference Suite DVD]] . 2009.</ref>

===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> Muscle movements have been described as among the fastest in [[metazoan]]s.<ref name="Perez2021"/> Muscles are directly excitable by electrical currents or strong [[potassium|K+]] solutions; the main neuromuscular transmitter is acetylcholine.<ref name="Perez2021"/>