Les ''échinidés'' ou ''echinides'' (surnommés '''oursins''', '''hérisson de mer''' ou '''châtaigne de mer''') sont des organismes marins portant des piquants (non des épines) sur l'entièreté du corps. 

==Morphologie==

<gallery>
Image:Diadema setosum 2.JPG|
Image:Paracentrotus lividus.001 - Aquarium Finisterrae.JPG|
Image:Kråkeboller.jpg|
Image:Sea urchin tests.jpg|
</gallery>

La couleur de l'animal est très variable, elle peut être brune, noire, pourpre, verte, blanche, rouge ou multicolore. 
Beaucoup d'espèces présentent des tailles allant de 6 à 12 cm de diamètre, mais certaines espèces du Pacifique peuvent atteindre des diamètres de 36 cm. Comme le reste des membres du Phylum des échinodermes les oursins présentent un corps à structure dite pentamèrique (symétrie d'ordre 5). L'animal ne présente donc pas une face ventrale et dorsale mais une face aborale (anus) et un face orale (bouche). Chez les oursins dit réguliers la face aborale est située sur le dessus de l'animal et la face orale sur le dessous et directement en contact avec le substrat (oursins irréguliers). Le corps globuleux de l'animal peut être divisé en  10 sections radiaires qui convergent vers le pôle oral et le pôle aboral. 
On peut observer 5 sections portant des podia ou podions qui se nomment les zones ambulacraires.

Ce sont des concombres de mer et des étoiles de mer.

L'oursin (châtaigne des mers) commun du littoral européen se nomme ''Paracentrotus lividus''.

==Systématique==
===Place des oursins dans le monde animal===

===Liste des sous-classes===
Selon ITIS:
* sous-classe ''Euechinoidea'' Bronn, 1860
** super-ordre ''Atelostomata'' Zittel, 1879
*** ordre ''Cassiduloida'' Claus, 1880
*** ordre ''Spatangoida'' Claus, 1876
** super-ordre ''Diadematacea'' Duncan, 1889
*** ordre ''Diadematoida'' Duncan, 1889
*** ordre ''Echinothurioida'' Claus, 1880
*** ordre ''Pedinoida''
** super-ordre ''Echinacea'' Claus, 1876
*** ordre ''Arbacioida'' Gregory, 1900
*** ordre ''Echinoida'' Claus, 1876
*** ordre ''Phymosomatoida''
*** ordre ''Salenioida''
*** ordre ''Temnopleuroida'' Mortensen, 1942
** super-ordre ''Gnathostomata'' Zittel, 1879
*** ordre ''Clypeasteroida'' A. Agassiz, 1872
*** ordre ''Holectypoida'' Ducan, 1889
* sous-classe ''Perischoechinoidea'' M'coy, 1849
** ordre ''Cidaroida'' Claus, 1880

==Oursin et l'Homme==
=== Consommation d'oursin===
<gallery>
Image:Fish0889.jpg|Oursin recoltés pour la consommation. 
Image:Sea urchin eggs.jpg|Détail des oeufs d'un oursin pret à la consommation.
</gallery>

Tous les oursins ne sont pas consommables. Avec des ciseaux pointus, on découpe à mi-hauteur la carapace, en partant de la zone molle dépourvue de piquants autour de la bouche. On consomme les cinq glandes sexuelles c'est-à-dire le corail et le liquide qui les entourent. Pour y avoir accès, il faut enlever la bouche et l'appareil digestif (la lanterne d'Aristote). On mange le corail cru. Suivant la saison, le corail est plus ou moins aggloméré et sa couleur va de verdâtre à rouge sombre. L'oursin est aussi appellé "oeuf de Mer " (Victor Hugo in "Les travailleurs de la mer").

Ils sont consommés dans de nombreux pays du monde. La pêche et la vente sont interdites de mai à septembre en France

{{Taxobox
| name = Sea urchin
| image = Strongylocentrotus purpuratus P1160330.jpg
| image_width = 250px
| image_caption = The sea urchin ''Strongylocentrotus purpuratus'' in an aquarium
| regnum = Animalia
| phylum = Echinodermata
| subphylum = Echinozoa
| classis = '''Echinoidea'''
| classis_authority=Leske, 1778
| subdivision_ranks = Subclasses
| subdivision = 
*Subclass Perischoechinoidea
**Order Cidaroida (pencil urchins)
*Subclass Euechinoidea
**Superorder Atelostomata
***Order Cassiduloida
***Order Spatangoida (heart urchins)
**Superorder Diadematacea
***Order Diadematoida
***Order Echinothurioida
***Order Pedinoida
**Superorder Echinacea
***Order Arbacioida
***Order Echinoida
***Order Phymosomatoida
***Order Salenioida
***Order Temnopleuroida
**Superorder Gnathostomata
***Order Clypeasteroida (sand dollars)
***Order Holectypoida
}}

'''Sea urchins''' are small, globular, spiny sea creatures, composing most of class '''Echinoidea'''. They are found in oceans all over the world.  Their shell, or "test", is round and spiny, typically from 3 to 10 cm across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae. Otters, wolf eels, and other predators feed on urchins. Sea urchins are harvested and served as a delicacy.

Sea urchins are members of the phylum Echinodermata, which also includes pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The pentamerous symmetry is not obvious at a casual glance but is easily seen in the dried shell or test of the urchin.

Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is defined by primarily having a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved different shapes. Although many sea cucumbers have branched tentacles surrounding the oral opening, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars (starfish) and brittle stars.

==Classification and nomenclature==
Within the echinoderms, sea urchins are classified as '''echinoids''' (class Echinoidea). Specifically, the term "sea urchin" refers to the "regular echinoids," which are symmetrical and globular. The ordinary phrase "sea urchin" actually includes several different taxonomic groups: the Echinoida and the Cidaroida or "slate-pencil urchins", which have very thick, blunt spines (see image at right), and others (see taxonomic box on the right). Besides sea urchins, the Echinoidea also includes three groups of "irregular" echinoids: flattened sea biscuits, and heart urchins.

The name ''urchin'' is an old name for the round spiny hedgehogs that sea urchins resemble.

==Physiology==
At first glance, a sea urchin often appears sessile, i.e. incapable of moving. Sometimes the most visible sign of life is the spines, which are attached at their bases to ball-and-socket joints and can be pointed in any direction. In most urchins, a light touch elicits a prompt and visible reaction from the spines, which converge toward the point that has been touched. A sea urchin has no visible eyes, legs, or means of propulsion, but it can move freely over surfaces by means of its adhesive tube feet, working in conjunction with its spines.   

On the oral surface of the sea urchin is a centrally located mouth made up of five united calcium carbonate teeth or jaws, with a fleshy tongue-like structure within.  The entire chewing organ is known as Aristotle's lantern, which name comes from Aristotle's accurate description in his ''History of Animals'':

:…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 anal vent, where the shell is perforated for an outlet... 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. (Tr. D'Arcy Thompson)

The spines, which in some species are long and sharp, serve to protect the urchin from predators. The spines can inflict a painful wound on a human who steps on one, but they are not seriously dangerous, and it is not clear that the spines are truly venomous (unlike the pedicellariae between the spines, which are venomous).

Typical sea urchins have spines that are 1 to 3 cm in length, 1 to 2 mm thick, and not terribly sharp. ''Diadema antillarum'', familiar in the Caribbean, has thin, potentially dangerous spines that can be 10 to 20 cm long. 

==Diet==
thumb|''[[Echinothrix calamaris'', a species of sea urchin. The sphere in the middle of a sea urchin is its anus]]
Sea urchins feed mainly on wolf eels. Left unchecked, urchins will devastate their environment, creating what biologists call an urchin barren, devoid of macroalgae and associated fauna. Where sea otters have been re-introduced into British Columbia, the health of the coastal ecosystem has improved dramatically.<ref name=dfo></ref>

==Geologic history==
thumb|left|150px|Fossil sea urchin ''Lovenia woodsi'' from the  [[Pliocene of Australia.]]
The earliest known echinoids are found in the rock of the upper part of the MYA), and they have survived to the present day, where they are a successful and diverse group of organisms. In well-preserved specimens the spines may be present, but usually only the test is found. Sometimes isolated spines are common as fossils. Some echinoids (such as ''Tylocidaris clavigera'', which is found in the Cretaceous period Chalk Formation of England) had very heavy club-shaped spines that would be difficult for an attacking predator to break through and make the echinoid awkward to handle. Such spines are also good for walking on the soft sea-floor.

thumb|right|150px|[[Cretaceous echinoids from Castle Hayne quarry, North Carolina, USA]]
Complete fossil echinoids from the Paleozoic era are generally rare, usually consisting of isolated spines and small clusters of scattered plates from crushed individuals. Most specimens occur in rocks from the Devonian and Carboniferous periods. The shallow water limestones from the Ordovician and Silurian periods of Estonia are famous for the echinoids found there. The Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin test, they would certainly not have survived in the turbulent wave-battered coastal waters inhabited by many modern echinoids today. During the upper part of the Carboniferous period, there was a marked decline in echinoid diversity, and this trend continued into the Permian period. They neared extinction at the end of the Paleozoic era, with just six species known from the Permian period. Only two separate lineages survived the massive extinction of this period and into the Triassic: the genus ''Miocidaris'', which gave rise to the modern cidaroids (pencil urchins), and the ancestor that gave rise to the euechinoids. By the upper part of the Triassic period, their numbers began to increase again. The cidaroids have changed very little since their modern design was established in the Late Triassic and are today considered more or less as living fossils. 
The euechinoids, on the other hand, diversified into new lineages throughout the Jurassic period and into the Cretaceous period, and from them emerged the first irregular echinoids (superorder Atelostomata) during the early Jurassic, and when including the other superorder (Gnathostomata) or irregular urchins which evolved independently later, they now represent 47% of all present species of echinoids thanks to their adaptive breakthroughs in both habit and feeding strategy, which allowed them to exploit habitats and food sources unavailable to regular echinoids. During the Mesozoic and Cenozoic eras the echinoids flourished. While most echinoid fossils are restricted to certain localities and formations, where they do occur, they are quite often abundant. An example of this is ''Enallaster'', which may be collected by the thousands in certain outcrops of limestone from the Cretaceous period in Texas. Many fossils of the Late Jurassic ''Plesiocidaris'' still have the spines attached.

Some echinoids, such as ''Micraster'' which is found in the Cretaceous period index fossils. Because they evolved rapidly over time, such fossils are useful in enabling geologists to date the rocks in which they are found. However, most echinoids are not abundant enough and may be too limited in their geographic distribution to serve as zone fossils.

In the early Tertiary (''c'' 65 to 1.8 MYA), sand dollars (order Clypeasteroida) arose. Their distinctive flattened test and tiny spines were adapted to life on or under loose sand. They form the newest branch on the echinoid tree.

==Culinary Usage/Fishery==
Sea urchins are an important fishery and are harvested for food. Contrary to popular belief, the portion of the sea urchin sold and served as one of the ocean’s most opulent treasures is not the roe. It is the gonads of this hermaphrodite sea creature that are scooped out of the urchin’s spiny shell in five custard-like, golden sections. Known in Japan as "uni" and traditionally considered an aphrodisiac, gonads are the only edible part of the urchin.