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sea urchin /ˈsiː ɜː.tʃɪn/
a small sea creature with a round shell which is covered with spikes, common name for approximately 700 species of echinoderms having rigid, spherical shells, or tests, made up of closely fitting bony plates. They move about by means of short to long, movable spines. The mouth is at one pole of the sphere, and the anus is at the opposite pole. Scattered over the surface of the test are microscopic, buttonlike bodies called sphaeridia, which are thought to be organs of balance; between the spines are small, pincerlike organs called pedicellariae, used for cleaning and defense. Sea urchins also have a complicated chewing apparatus called Aristotle's lantern, consisting of five converging jaws and accessory bony plates.
Sea urchins feed on various types of organic matter, including plants, small animals, and waste material. Some species, called heart urchins, lack jaws and burrow into sand or mud for their food. A number of species are venomous to humans, and some species are used as food in East Asia and South America. Primitive sea urchins do not have gills about the mouth; other sea urchins have gills.
海膽屬於棘皮動物門(Echinodermata)海膽綱(Echinoidea)體呈球形的動物，約700種。表面骨板互相嵌合成殼。從口到肛門有5條步帶區，上有孔。管足從孔伸出，細而末端通常有吸盤。節上有可動的長棘和叉棘，上面可能有毒腺。口位於體下面，內有稱作 「亞裡斯多德氏提燈」 的一種複雜的咀嚼器官，也可能有毒，有齒，能伸出從巖石上刮取藻類和其他食物。有些種能在珊瑚、巖石，甚至鋼鐵中開鑿隱蔽場所。生活在海底，常在硬物上，用管足和棘運動。內部器官亦輻射排列。