Editing Sea cucumber (section)

==Anatomy==
Sea cucumbers are typically {{cvt|10|to|30|cm|in|0}} in length, although the smallest known species are just {{cvt|3|mm|in}} long, and [[Largest organisms#Echinoderms (Echinodermata)|the largest]] can reach {{convert|3|m|ft|sp=us|0}}. The body ranges from almost spherical to worm-like, and lacks the arms found in many other echinoderms, such as [[starfish]]. The anterior end of the animal, containing the mouth, corresponds to the oral pole of other echinoderms (which, in most cases, is the underside), while the posterior end, containing the anus, corresponds to the aboral pole. Thus, compared with other echinoderms, sea cucumbers can be said to be lying on their side.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher=Holt-Saunders International |location= Philadelphia, PA|pages=981–997|isbn= 978-0-03-056747-6}}</ref>
[[File:Conspicuous Sea Cucumber, Coconut Island, Hawaii.JPG|thumb|Conspicuous Sea Cucumber, [[Coconut Island (Hawaii Island)|Coconut Island]], Hawaii]]

===Body plan===
The body of a holothuroid is roughly cylindrical. It is radially symmetrical along its longitudinal axis, and has weak bilateral symmetry transversely with a dorsal and a ventral surface. As in other [[echinozoa]]ns, there are five [[Ambulacral|ambulacra]] separated by five ambulacral grooves, the interambulacra. The ambulacral grooves bear four rows of tube feet but these are diminished in size or absent in some holothuroids, especially on the dorsal surface. The two dorsal ambulacra make up the bivium while the three ventral ones are known as the trivium.<ref name=Lander>[http://webs.lander.edu/rsfox/invertebrates/cucumaria.html ''Cucumaria frondosa''] {{webarchive|url=https://web.archive.org/web/20110903211033/http://webs.lander.edu/rsfox/invertebrates/cucumaria.html |date=2011-09-03 }} Invertebrate anatomy online. ed2012-02-12.</ref>

At the anterior end, the mouth is surrounded by a ring of tentacles which are usually retractable into the mouth. These are called the primary tentacles and were present in the common ancestor of echinoderms, but have been lost in all the other classes of the phylum,<ref>{{cite journal | url=https://link.springer.com/article/10.1134/S0031030122080020 | doi=10.1134/S0031030122080020 | title=Origin of Echinodermata | date=2022 | last1=Ezhova | first1=O. V. | last2=Malakhov | first2=V. V. | journal=Paleontological Journal | volume=56 | issue=8 | pages=938–973 | bibcode=2022PalJ...56..938E | url-access=subscription }}</ref> and may be simple, branched or arborescent. They are known as the introvert and posterior to them there is an internal ring of large calcareous ossicles. Attached to this are five bands of muscle running internally longitudinally along the ambulacra. There are also circular muscles, contraction of which cause the animal to elongate and the introvert to extend. Anterior to the ossicles lie further muscles, contraction of which cause the introvert to retract.<ref name=Lander/>

The body wall consists of an epidermis and a dermis and contains smaller calcareous ossicles, the types of which are characteristics which help to identify different species. Inside the body wall is the coelom which is divided by three longitudinal [[Mesentery (zoology)|mesenteries]] which surround and support the internal organs.<ref name=Lander/>

===Digestive system===
[[File:Holothuroidea (Sea cucumber feeding).jpg|thumb|right|A sea cucumber atop gravel, feeding]]
A [[pharynx]] lies behind the mouth and is surrounded by a ring of ten [[calcareous]] plates. In most sea cucumbers, this is the only substantial part of the skeleton, and it forms the point of attachment for muscles that can retract the tentacles into the body for safety as for the main muscles of the body wall. Many species possess an [[oesophagus]] and [[stomach]], but in some the pharynx opens directly into the [[intestine]]. The intestine is typically long and coiled, and loops through the body three times before terminating in a [[cloaca]]l chamber, or directly as the [[anus]].<ref name=IZ/>

===Nervous system===
Sea cucumbers have no true [[brain]]. A ring of neural tissue surrounds the oral cavity, and sends nerves to the tentacles and the [[pharynx]]. The animal is, however, quite capable of functioning and moving about if the nerve ring is surgically removed, demonstrating that it does not have a central role in nervous coordination. In addition, five major nerves run from the nerve ring down the length of the body beneath each of the ambulacral areas.<ref name=IZ/>

Most sea cucumbers have no distinct sensory organs, although there are various nerve endings scattered through the skin, giving the animal a sense of touch and a sensitivity to the presence of light. There are, however, a few exceptions: members of the Apodida order are known to possess [[statocyst]]s, while some species possess small eye-spots near the bases of their tentacles.<ref name=IZ/>

===Respiratory system===
Sea cucumbers use [[cloacal respiration]] via a pair of "respiratory trees" that branch in the [[cloaca]] just inside the [[anus]], so that they "breathe" by drawing water in through the anus, extracting [[dissolved oxygen]] from water, and then expelling it.<ref>{{cite web | url = http://www.fegi.ru/prim/sea/golot.htm | title = Holothurians or sea cucumbers | access-date = 2007-10-03 | archive-date = 2007-09-30 | archive-url = https://web.archive.org/web/20070930154932/http://www.fegi.ru/prim/sea/golot.htm | url-status = dead }}</ref><ref>{{cite web | url = http://www.comoxvalleynaturalist.bc.ca/knowing_nature/2006/sea_cucumbers.html | title = Knowing Nature... Cool as a Sea Cucumber | access-date = 2007-10-03 | last = Ingram | first = Jocie | date = 2006-06-16 | url-status = dead | archive-url = https://web.archive.org/web/20071021183031/http://www.comoxvalleynaturalist.bc.ca/knowing_nature/2006/sea_cucumbers.html | archive-date = 2007-10-21 }}</ref> The "trees" consist of a series of narrow tubules branching from a common duct, and lie on either side of the [[digestive tract]]. [[Gas exchange]] occurs across the thin walls of the tubules, to and from the fluid of the main body cavity.

Together with the intestine, the "respiratory trees" also act as [[excretory]] organs, with nitrogenous waste diffusing across the tubule walls in the form of [[ammonia]] and [[phagocytosis|phagocytic]] [[coelomocyte]]s depositing particulate waste.<ref name=IZ/>

===Circulatory systems===
Like all echinoderms, sea cucumbers possess both a [[water vascular system]] that provides hydraulic pressure to the tentacles and tube feet, allowing them to move, and a ''haemal system''. The latter is more complex than that in other echinoderms, and consists of well-developed vessels as well as open [[Hemocoel|sinuses]].<ref name=IZ/>

A central haemal ring surrounds the pharynx next to the ring canal of the water vascular system, and sends off additional vessels along the radial canals beneath the ambulacral areas. In the larger species, additional vessels run above and below the intestine and are connected by over a hundred small muscular ampullae, acting as miniature hearts to pump blood around the haemal system. Additional vessels surround the respiratory trees, although they contact them only indirectly, via the [[Coelom#Coelomic fluid|coelomic fluid]].<ref name=IZ/>

Indeed, the blood itself is essentially identical with the coelomic fluid that bathes the organs directly, and also fills the water vascular system. Phagocytic coelomocytes, somewhat similar in function to the [[white blood cell]]s of [[vertebrates]], are formed within the haemal vessels, and travel throughout the body cavity as well as both circulatory systems. An additional form of coelomocyte, not found in other echinoderms, has a flattened discoid shape, and contains [[hemoglobin]]. As a result, in many (though not all) species, both the blood and the coelomic fluid are red in colour.<ref name=IZ/>

{{multiple image
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 | direction = vertical
 | width     = 220
 | image1    = Pearsonothuria graeffei bouche.JPG
 | alt1      = 
 | caption1  = ''[[Pearsonothuria graeffei]]'' showing its three rows of podia on its ''trivium''
 | image2    = Zeekomkommer-insluitselen.jpg
 | alt2      = 
 | caption2  = Sea cucumber ossicles (here "wheels" and "anchors")
}}

[[Vanadium]] has been reported in high concentrations in holothuroid blood,<ref>{{cite book | last = Phillips | first = Alexander |title = A Possible Source of Vanadium in Sedimentary Rocks | publisher =Princeton University | year = 1918 | url={{google books |plainurl=y |id=O6KxwrrC_LMC|page=473}}|page=473}}</ref> however researchers have been unable to reproduce these results.<ref>{{cite journal | last1 = Ciereszko | first1 = L. |title = On the Occurrence of Vanadium in Holothurians | year = 1962 |doi=10.1016/0010-406X(62)90034-8  |last2 = Ciereszko | first2 = E | last3 = Harris | first3 = E | last4 =Lane | first4 = C | journal = Comparative Biochemistry and Physiology |volume = 7 | issue = 1–2 | pages = 127–9 | pmid = 14021342}}</ref>

=== Locomotive organs ===
Like all echinoderms, sea cucumbers possess [[pentaradial symmetry]], with their bodies divided into five nearly identical parts around a central axis. However, because of their posture, they have secondarily evolved a degree of bilateral symmetry. For example, because one side of the body is typically pressed against the substratum, and the other is not, there is usually some difference between the two surfaces (except for [[Apodida]]). Like [[sea urchin]]s, most sea cucumbers have five strip-like ambulacral areas running along the length of the body from the mouth to the anus. The three on the lower surface have numerous [[tube feet]], often with suckers, that allow the animal to crawl along; they are called ''trivium''. The two on the upper surface have under-developed or vestigial tube feet, and some species lack tube feet altogether; this face is called ''bivium''.<ref name=IZ/>

In some species, the ambulacral areas can no longer be distinguished, with tube feet spread over a much wider area of the body. Those of the order [[Apodida]] have no tube feet or ambulacral areas at all, and burrow through sediment with muscular contractions of their body similar to that of worms, however five radial lines are generally still obvious along their body.<ref name=IZ/>

Even in those sea cucumbers that lack regular tube feet, those that are immediately around the mouth are always present. These are highly modified into retractile [[tentacle]]s, much larger than the locomotive tube feet.  Depending on the species, sea cucumbers have between 10 and 30 such tentacles and these can have a wide variety of shapes depending on the diet of the animal and other conditions.<ref name=IZ/>

Many sea cucumbers have papillae, conical fleshy projections of the body wall with sensory tube feet at their apices.<ref>{{cite web|url=http://species-identification.org/species.php?species_group=nasc&selected=definitie&menuentry=woordenlijst&record=PapillaMarine|title=Marine Species Identification Portal : North Australian Sea Cucumbers : Glossary : PapillaMarine|access-date=12 June 2015}}</ref> These can even evolve into long antennae-like structures, especially on the abyssal genus ''[[Scotoplanes]]''.

=== Endoskeleton ===
Echinoderms typically possess an internal skeleton composed of plates of [[calcium carbonate]] within the [[dermis]]. In most sea cucumbers, however, these have become reduced to microscopic ossicles embedded beneath the skin. A few genera, such as ''[[Sphaerothuria]]'', retain relatively large plates, giving them a scaly armour.<ref name=IZ/>