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Box jellyfish
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== Description == [[File:Haeckel Cubomedusae.jpg|thumb|150px|"Cubomedusae", from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'', 1904]] The medusa form of a box jellyfish has a squarish, box-like bell, from which its name is derived. From each of the four lower corners of this hangs a short pedalium or stalk which bears one or more long, slender, hollow [[tentacle]]s. The rim of the bell is folded inwards to form a shelf known as a velarium which restricts the bell's aperture and creates a powerful jet when the bell pulsates.<ref name=Ruppert>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=153–154 }}</ref> As a result, box jellyfish can move more rapidly than other jellyfish; speeds of up to 1.5 metres per second have been recorded.<ref name=IZ>{{Cite book|author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages=139–149|isbn= 0-03-056747-5}}</ref> In the center of the underside of the bell is a mobile appendage called the manubrium which somewhat resembles an elephant's trunk. At its tip is the mouth. The interior of the bell is known as the [[gastrovascular cavity]]. It is divided by four equidistant septa into a central stomach and four gastric pockets. The eight [[gonad]]s are located in pairs on either side of the four septa. The margins of the septa bear bundles of small gastric filaments which house [[nematocyst]]s and digestive glands and help to subdue [[Predation|prey]]. Each septum is extended into a septal funnel that opens onto the oral surface and facilitates the flow of fluid into and out of the animal.<ref name=Ruppert/> [[File:Cubozoan visual system in Tripedalia cystophora.png|thumb|400px|The complex [[rhopalia]]l [[ocelli]] of ''[[Tripedalia cystophora]]'' ]] The box jellyfish's nervous system is more developed than that of many other jellyfish. They possess a ring nerve at the base of the bell that coordinates their pulsing movements, a feature found elsewhere only in the [[crown jellyfish]]. Whereas some other jellyfish have simple pigment-cup [[Simple eye in invertebrates|ocelli]], box jellyfish are unique in the possession of true eyes, complete with [[retina]]s, [[cornea]]s and [[lens (anatomy)|lenses]].<ref>{{Cite journal |last1=Nilsson |first1=Dan-E. |last2=Gislén |first2=Lars |last3=Coates |first3=Melissa M. |last4=Skogh |first4=Charlotta |last5=Garm |first5=Anders |date=May 2005 |title=Advanced optics in a jellyfish eye |url=https://www.nature.com/articles/nature03484 |journal=Nature |language=en |volume=435 |issue=7039 |pages=201–205 |doi=10.1038/nature03484 |issn=1476-4687|url-access=subscription }}</ref> Their eyes are set in clusters at the ends of sensory structures called [[Rhopalium|rhopalia]] which are connected to their ring nerve. Each rhopalium contains two image-forming lens eyes. The upper lens eye looks straight up out of the water with a field of view that matches [[Snell's window]]. In species such as ''[[Tripedalia cystophora]]'', the upper lens eye is used to navigate to their preferred habitats at the edges of mangrove lagoons by observing the direction of the tree canopy.<ref name=":1">{{Cite journal |last1=Garm |first1=Anders |last2=Oskarsson |first2=Magnus |last3=Nilsson |first3=Dan-Eric |date=2011-05-10 |title=Box Jellyfish Use Terrestrial Visual Cues for Navigation |journal=Current Biology |language=en |volume=21 |issue=9 |pages=798–803 |doi=10.1016/j.cub.2011.03.054 |issn=0960-9822|doi-access=free }}</ref> The lower lens eye is primarily used for object avoidance. Research has shown that the minimum visual angle for obstacles avoided by their lower lens eyes matches the half-widths of their receptive fields.<ref>{{Cite journal |last1=Garm |first1=A |last2=O'Connor |first2=M |last3=Parkefelt |first3=L |last4=Nilsson |first4=D |date=October 15, 2007 |title=Visually guided obstacle avoidance in the box jellyfish Tripedalia cystophora and Chiropsella bronzie |url=https://journals.biologists.com/jeb/article/210/20/3616/17139/Visually-guided-obstacle-avoidance-in-the-box |journal=Journal of Experimental Biology |volume=210 |issue=20}}</ref> Each rhopalium also has two pit eyes on either side of the upper lens eye which likely act as mere light meters, and two slit eyes on either side of the lower lens eye which are likely used to detect vertical movement.<ref>{{Cite journal |last1=Garm |first1=A. |last2=Andersson |first2=F. |last3=Nilsson |first3=Dan-E. |date=2008-03-01 |title=Unique structure and optics of the lesser eyes of the box jellyfish Tripedalia cystophora |journal=Vision Research |language=en |volume=48 |issue=8 |pages=1061–1073 |doi=10.1016/j.visres.2008.01.019 |issn=0042-6989|doi-access=free }}</ref> In total, the box jellyfish have six eyes on each of their four rhopalia, creating a total of 24 eyes. The rhopalia also feature a heavy crystal-like structure called a statolith, which, due to the flexibility of the rhopalia, keep the eyes oriented vertically regardless of the orientation of the bell.<ref name=":1" /> Box jellyfish also display complex, probably visually-guided behaviors such as obstacle avoidance and fast directional swimming.<ref name="ncbi.nlm.nih.gov">{{cite journal | vauthors = Skogh C, Garm A, Nilsson DE, Ekström P | title = Bilaterally symmetrical rhopalial nervous system of the box jellyfish Tripedalia cystophora | journal = Journal of Morphology | volume = 267 | issue = 12 | pages = 1391–405 | date = December 2006 | pmid = 16874799 | doi = 10.1002/jmor.10472 }}</ref> Research indicates that, owing to the number of rhopalial nerve cells and their overall arrangement, visual processing and integration at least partly happen within the rhopalia of box jellyfish.<ref name="ncbi.nlm.nih.gov"/> The complex nervous system supports a relatively advanced sensory system compared to other jellyfish, and box jellyfish have been described as having an active, fish-like behavior.<ref>{{cite journal | vauthors = Nilsson DE, Gislén L, Coates MM, Skogh C, Garm A | title = Advanced optics in a jellyfish eye | journal = Nature | volume = 435 | issue = 7039 | pages = 201–5 | date = May 2005 | pmid = 15889091 | doi = 10.1038/nature03484 | bibcode = 2005Natur.435..201N | s2cid = 4418085 }}</ref> Depending on species, a fully grown box jellyfish can measure up to {{convert|20|cm|in|0|abbr=on}} along each box side ({{convert|30|cm|in|abbr=on|disp=or}} in diameter), and the tentacles can grow up to {{convert|3|m|ft|0|abbr=on}} in length. Its weight can reach {{convert|2|kg|lb|frac=2|abbr=on}}.<ref name="National Geographic Online">{{cite web|url=http://animals.nationalgeographic.com/animals/invertebrates/box-jellyfish|archive-url=https://web.archive.org/web/20100124062101/http://animals.nationalgeographic.com/animals/invertebrates/box-jellyfish/|url-status=dead|archive-date=January 24, 2010|url-access=registration|title=Box Jellyfish, Box Jellyfish Pictures, Box Jellyfish Facts|date=10 September 2010 |publisher=NationalGeographic.com|access-date=2012-08-27}}</ref> However, the [[Nail (anatomy)|thumbnail]]-sized [[Irukandji_jellyfish|Irukandji]] is a box jellyfish, and lethal despite its small size. There are about 15 tentacles on each corner. Each tentacle has about 500,000 [[cnidocyte]]s, containing [[Cnidocyte|nematocyst]]s, a harpoon-shaped microscopic mechanism that injects venom into the victim.<ref name="1996-Williamson">{{Cite book|title=Venomous and poisonous marine animals: a medical and biological handbook|publisher=Surf Life Saving Australia and University of New North Wales Press Ltd|year=1996|isbn=0-86840-279-6|veditors=Williamson JA, Fenner PJ, Burnett JW, Rifkin J}}{{page needed|date=May 2014}}</ref> Many different kinds of nematocysts are found in cubozoans.<ref>{{cite journal|last=Gershwin|first=L|year=2006|title=Nematocysts of the Cubozoa|url=http://www.mapress.com/zootaxa/2006f/z01232p057f.pdf|journal=Zootaxa|issue=1232|pages=1–57}}</ref>
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