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Sea urchins or urchins (), archaically called sea hedgehogs,〔Wright, Anne. 1851. ''The Observing Eye, Or, Letters to Children on the Three Lowest Divisions of Animal Life.'' London: Jarrold and Sons, p. 107.〕〔Soyer, Alexis. 1853. ''The Pantropheon Or History Of Food, And Its Preparation: From The Earliest Ages Of The World.'' Boston: Ticknor, Reed, and Fields,, p. 245.〕 are small, spiny, globular animals that, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. About 950 species of echinoids inhabit all oceans from the intertidal to 5000 m deep.〔(【引用サイトリンク】title=Animal Diversity Web – Echinoidea )〕 The shell, or "test", of sea urchins is round and spiny, typically from across. Common colors include black and dull shades of green, olive, brown, purple, blue, and red. Sea urchins move slowly, and feed on mostly algae. Sea otters, starfish, wolf eels, triggerfish, and other predators hunt and feed on sea urchins. Their roe is a delicacy in many cuisines. The name "urchin" is an old word for hedgehog, which sea urchins resemble. == Taxonomy == Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms, they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The symmetry is not obvious in the living animal, but is easily visible in the dried test. Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, including two subclasses : Euechinoidea ("modern" sea urchins, including irregular ones) and Cidaroidea or "slate-pencil urchins", which have very thick, blunt spines, with algae and sponges growing on it. The irregular sea urchins are an infra-classis inside the Euechinoidea, called Irregularia, and including Atelostomata and Neognathostomata. "Irregular" echinoids include: flattened sand dollars, sea biscuits, and heart urchins. Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. Although many sea cucumbers have branched tentacles surrounding their oral openings, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, and brittle stars. File:Paracentrotus lividus profil.JPG|''Paracentrotus lividus'', a regular sea urchin (Euechinoidea, infraclass Carinacea). File:Live Sand Dollar trying to bury itself in beach sand.jpg|A sand dollar, an irregular sea urchin (Irregularia). File:Phyllacanthus.jpg|''Phyllacanthus imperialis'', a cidaroid sea urchin (Cidaroidea). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sea urchin」の詳細全文を読む スポンサード リンク
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