Editing Sea urchin (section)

== Systems ==
=== Musculoskeletal ===
{{Further|Test (biology)|Tube feet}}
[[File:Seeigel-Saugfuesse(Galicien2005).jpg|thumb|upright=0.7|[[Tube feet]] of a [[strongylocentrotus purpuratus|purple sea urchin]]]]

The internal organs are enclosed in a hard shell or test composed of fused plates of [[calcium carbonate]] covered by a thin [[dermis]] and [[epidermis]].  The test is referred to as an [[endoskeleton]] rather than exoskeleton even though it encloses almost all of the urchin. This is because it is covered with a thin layer of muscle and skin; sea urchins also do not need to molt the way invertebrates with true exoskeletons do, instead the plates forming the test grow as the animal does.

The test is rigid, and divides into five ambulacral grooves separated by five wider interambulacral areas. Each of these ten longitudinal columns consists of two sets of plates (thus comprising 20 columns in total). The  ambulacral plates have pairs of tiny holes through which the tube feet extend.<ref>{{cite web |title=The Echinoid Directory – Natural History Museum |url=https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/intro.html |website=www.nhm.ac.uk |publisher=Natural History Museum, UK |access-date=28 December 2022 |archive-date=8 December 2023 |archive-url=https://web.archive.org/web/20231208134929/https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/intro.html |url-status=live }}</ref>

All of the plates are covered in rounded tubercles to which the spines are attached. The spines are used for defence and for locomotion and come in a variety of forms.<ref>{{cite web |title=The Echinoid Directory – Natural History Museum |url=https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/intro.html |website=www.nhm.ac.uk |publisher=Natural History Museum, UK. |access-date=28 December 2022 |archive-date=8 December 2023 |archive-url=https://web.archive.org/web/20231208134929/https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/intro.html |url-status=live }}</ref> The inner surface of the test is lined by [[peritoneum]].<ref name=IZ /> Sea urchins convert aqueous [[carbon dioxide]] using a [[catalytic]] process involving [[nickel]] into the calcium carbonate portion of the test.<ref>{{cite web|url=http://www.gizmag.com/carbon-capture-calcium-carbonate/26101/|title=Sea urchins reveal promising carbon capture alternative|publisher=Gizmag|date=4 February 2013|access-date=2013-02-05|archive-date=2016-07-14|archive-url=https://web.archive.org/web/20160714210351/http://www.gizmag.com/carbon-capture-calcium-carbonate/26101/|url-status=live}}</ref>

[[File:Ricci di mare luminosi.jpg|thumb|Mediterranean sea urchins illuminated to reveal the mesodermal  calcite structure.]]

Most species have two series of spines, primary (long) and secondary (short), distributed over the surface of the body, with the shortest at the poles and the longest at the equator. The spines are usually hollow and cylindrical. Contraction of the muscular sheath that covers the test causes the spines to lean in one direction or another, while an inner sheath of collagen fibres can reversibly change from soft to rigid which can lock the spine in one position. Located among the spines are several types of [[pedicellaria]], moveable stalked structures with jaws.<ref name=Ruppert/>

Sea urchins move by walking, using their many flexible tube feet in a way similar to that of starfish; regular sea urchins do not have any favourite walking direction.<ref>Kazuya Yoshimura, Tomoaki Iketani et Tatsuo Motokawa, "Do regular sea urchins show preference in which part of the body they orient forward in their walk ?", ''Marine Biology'', vol. 159, no 5, 2012, p. 959–965.</ref> The tube feet protrude through pairs of pores in the test, and are operated by a [[water vascular system]]; this works through [[hydraulic pressure]], allowing the sea urchin to pump water into and out of the tube feet. During locomotion, the tube feet are assisted by the spines which can be used for pushing the body along or to lift the test off the substrate. Movement is generally related to feeding, with the [[red sea urchin]] (''Mesocentrotus franciscanus'') managing about {{convert|7.5|cm|in|0|abbr=on}} a day when there is ample food, and up to {{convert|50|cm|in|0|abbr=on}} a day where there is not. An inverted sea urchin can right itself by progressively attaching and detaching its tube feet and manipulating its spines to roll its body upright.<ref name=Ruppert/> Some species bury themselves in soft sediment using their spines, and ''[[Paracentrotus lividus]]'' uses its jaws to burrow into soft rocks.<ref>{{cite book |last1=Boudouresque |first1=Charles F. |last2=Verlaque |first2=Marc |editor-last=Lawrence |editor-first=John M. |title=Edible Sea Urchins: Biology and Ecology |publisher=Elsevier |year=2006 |page=243 |chapter=13: Ecology of ''Paracentrotus lividus'' |chapter-url=https://books.google.com/books?id=6T2JomruARoC&pg=PA243 |isbn=978-0-08-046558-6 }}</ref>

<gallery style="text-align:center;" mode="packed">
File:Sea Urchin test 5629 03 22.jpg|Test of an ''[[Echinus esculentus]]'', a regular sea urchin
File:BlackSeaUrchinTest.jpg|Test of black sea urchin, showing tubercles and ambulacral plates (on right)
File:Inner surface of black sea urchin test.jpg|Inner surface of test, showing pentagonal interambulacral plates on right, and holes for tube feet on left.
File:Echinodiscus2.jpg|Test of an ''[[Echinodiscus tenuissimus]]'', an [[Irregularia|irregular sea urchin]] ("[[sand dollar]]")
File:Phyllacanthus imperialis test.JPG|Test of a ''[[Phyllacanthus imperialis]]'', a [[Cidaroida|cidaroid]] sea urchin. These are characterised by their big tubercles, bearing large radiola.
File:Sea urchin shell - pattern (6658690371).jpg|Close-up of the test showing an ambulacral groove with its two rows of pore-pairs, between two interambulacra areas (green). The tubercles are non-perforated.
File:Sea Urchin Shell detail.jpg|Close-up of a cidaroid sea urchin apical disc: the 5 holes are the gonopores, and the central one is the anus ("periproct"). The biggest genital plate is the [[madreporite]].<ref>{{cite web |title=Apical disc and periproct |url=https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/discmorph.html |publisher=[[Natural History Museum, London]] |access-date=2 November 2019 |archive-date=2 March 2024 |archive-url=https://web.archive.org/web/20240302122236/https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/discmorph.html |url-status=live }}</ref>
</gallery>

=== Feeding and digestion ===
[[File:Seeigel Gebiss.jpg|thumb|Dentition of a sea urchin]]

The mouth lies in the centre of the oral surface in regular urchins, or towards one end in irregular urchins. It is surrounded by lips of softer tissue, with numerous small, embedded bony pieces. This area, called the peristome, also includes five pairs of modified tube feet and, in many species, five pairs of gills.<ref name=IZ /> The jaw apparatus consists of five strong arrow-shaped plates known as pyramids, the ventral surface of each of which has a toothband with a hard tooth pointing towards the centre of the mouth. Specialised muscles control the protrusion of the apparatus and the action of the teeth, and the animal can grasp, scrape, pull and tear.<ref name=Ruppert/> The structure of the mouth and teeth have been found to be so efficient at grasping and grinding that similar structures have been tested for use in mechanical applications.<ref>{{Cite web |url=https://www.engadget.com/2016/05/04/sea-urchin-inspired-rover-claw/ |title=Claw inspired by sea urchins' mouth can scoop up Martian soil |access-date=2017-09-11 |archive-date=2019-10-12 |archive-url=https://web.archive.org/web/20191012025318/https://www.engadget.com/2016/05/04/sea-urchin-inspired-rover-claw/ |url-status=live }}</ref>

On the upper surface of the test at the aboral pole is a membrane, the [[periproct]], which surrounds the [[anus]]. The periproct contains a variable number of hard plates, five of which, the genital plates, contain the gonopores, and one is modified to contain the [[madreporite]], which is used to balance the water vascular system.<ref name=Ruppert/>

[[File:Lanterne d'Aristote.jpg|thumb|Aristotle's lantern in a sea urchin, viewed in lateral section]]

The mouth of most sea urchins is made up of five calcium carbonate teeth or plates, with a fleshy, tongue-like structure within. The entire chewing organ is known as Aristotle's lantern from [[Aristotle]]'s description in his ''[[History of Animals]]'' (translated by [[D'Arcy Thompson]]):

{{Blockquote|...&nbsp;the urchin has what we mainly call its head and mouth down below, and a place for the issue of the residuum up above. The urchin has, also, five hollow teeth inside, and in the middle of these teeth a fleshy substance serving the office of a [[tongue]]. Next to this comes the [[esophagus]], and then the [[stomach]], divided into five parts, and filled with excretion, all the five parts uniting at the [[anus|anal]] vent, where the shell is perforated for an outlet&nbsp;... In reality the mouth-apparatus of the urchin is continuous from one end to the other, but to outward appearance it is not so, but looks like a horn [[lantern]] with the panes of horn left out.}}

However, this has recently been proven to be a mistranslation. Aristotle's lantern is actually referring to the whole shape of sea urchins, which look like the ancient lamps of Aristotle's time.<ref>{{cite journal|last1=Voultsiadou|first1=Eleni|last2=Chintiroglou|first2=Chariton|title=Aristotle's lantern in echinoderms: an ancient riddle|url=http://users.auth.gr/~elvoults/pdf/Aristotle%27s%20lantern%2008.pdf|year=2008|volume=49|pages=299–302|journal=Cahiers de Biologie Marine|publisher=Station Biologique de Roscoff|issue=3|access-date=2020-12-23|archive-date=2020-12-23|archive-url=https://web.archive.org/web/20201223165338/http://users.auth.gr/~elvoults/pdf/Aristotle%27s%20lantern%2008.pdf|url-status=live}}</ref><ref>{{cite web|last1=Choi|first1=Charles Q.|title=Rock-Chewing Sea Urchins Have Self-Sharpening Teeth|url=https://news.nationalgeographic.com/news/2010/12/101228-sea-urchin-teeth-self-sharpening-tools-science-animals/|archive-url=https://web.archive.org/web/20101231160917/http://news.nationalgeographic.com/news/2010/12/101228-sea-urchin-teeth-self-sharpening-tools-science-animals/|url-status=dead|archive-date=December 31, 2010|website=National Geographic News|access-date=2017-11-12|date=29 December 2010}}</ref>

[[Heart urchin]]s are unusual in not having a lantern. Instead, the mouth is surrounded by [[cilia]] that pull strings of mucus containing food particles towards a series of grooves around the mouth.<ref name=IZ />

[[File:Echinoidea anatomie.svg|thumb|upright=1.1|Digestive and circulatory systems of a regular sea urchin:<br/>a = [[anus]]; m = [[madreporite]]; s = aquifer canal; r = radial canal; p = podial ampulla; k = test wall; i = [[intestine]]; b = mouth]]

The lantern, where present, surrounds both the mouth cavity and the [[pharynx]]. At the top of the lantern, the pharynx opens into the esophagus, which runs back down the outside of the lantern, to join the small [[intestine]] and a single [[caecum]]. The small intestine runs in a full circle around the inside of the test, before joining the large intestine, which completes another circuit in the opposite direction. From the large intestine, a [[rectum]] ascends towards the anus. Despite the names, the small and large intestines of sea urchins are in no way [[homology (biology)|homologous]] to the similarly named structures in vertebrates.<ref name=IZ />

Digestion occurs in the intestine, with the caecum producing further digestive [[enzyme]]s. An additional tube, called the siphon, runs beside much of the intestine, opening into it at both ends. It may be involved in resorption of water from food.<ref name=IZ />

=== Circulation and respiration ===
[[File:Diadema setosum (Kenya).JPG|thumb|''[[Diadema setosum]]'']]

The water vascular system leads downwards from the madreporite through the slender stone canal to the ring canal, which encircles the oesophagus. Radial canals lead from here through each ambulacral area to terminate in a small tentacle that passes through the ambulacral plate near the aboral pole. Lateral canals lead from these radial canals, ending in ampullae. From here, two tubes pass through a pair of pores on the plate to terminate in the tube feet.<ref name=Ruppert/>

Sea urchins possess a hemal system with a complex network of vessels in the mesenteries around the gut, but little is known of the functioning of this system.<ref name=Ruppert/> However, the main circulatory fluid fills the general body cavity, or [[coelom]]. This coelomic fluid contains [[phagocyte|phagocytic]] coelomocytes, which move through the vascular and hemal systems and are involved in internal transport and gas exchange. The coelomocytes are an essential part of [[blood clotting]], but also collect waste products and actively remove them from the body through the gills and tube feet.<ref name=IZ />

Most sea urchins possess five pairs of external gills attached to the peristomial membrane around their mouths. These thin-walled projections of the body cavity are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills' interiors by muscles associated with the lantern, but this does not provide a continuous flow, and occurs only when the animal is low in oxygen. Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills. The inside of each tube foot is divided by a septum which reduces diffusion between the incoming and outgoing streams of fluid.<ref name=Ruppert/>

=== Nervous system and senses ===
The nervous system of sea urchins has a relatively simple layout. With no true brain, the neural center is a large nerve ring encircling the mouth just inside the lantern. From the nerve ring, five nerves radiate underneath the radial canals of the water vascular system, and branch into numerous finer nerves to innervate the tube feet, spines, and [[pedicellariae]].<ref name=IZ />

Sea urchins are sensitive to touch, light, and chemicals. There are numerous sensitive cells in the epithelium, especially in the spines, pedicellaria and tube feet, and around the mouth.<ref name=Ruppert/> Although they do not have eyes or eye spots (except for [[Diadematidae|diadematids]], which can follow a threat with their spines), the entire body of most regular sea urchins might function as a compound eye.<ref>{{cite journal |last1=Knight |first1=K. |title=Sea Urchins Use Whole Body As Eye |journal=Journal of Experimental Biology |volume=213 |pages=i–ii |year=2009 |doi=10.1242/jeb.041715 |issue=2|doi-access=free }}
*{{cite press release |author=Charles Q. Choi |date=December 28, 2009 |title=Body of Sea Urchin is One Big Eye |website=[[LiveScience]] |url=http://www.livescience.com/animals/091228-sea-urchin-eye.html}}</ref> In general, sea urchins are negatively attracted to light, and seek to hide themselves in crevices or under objects. Most species, apart from [[Cidaris|pencil urchins]], have [[statocyst]]s in globular organs called spheridia. These are stalked structures and are located within the ambulacral areas; their function is to help in gravitational orientation.<ref name=IZ />