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Fossil of Acrocidaris, an extinct sea urchin. Echinoid fossils are the fossilised remains of sea urchins, spiny marine invertebrates that live on the seabed. Humans have been interested in these fossils for millennia, have considered them lucky, have imbued them with magical powers and linked them to their deities. [1]
The name urchin is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs. [6] [7] The name is derived from the Old French herichun, from Latin ericius ('hedgehog'). [8] Like other echinoderms, sea urchin early larvae have bilateral symmetry, [9] but they develop five-fold symmetry as they mature ...
Close-up of Asthenosoma varium, showing distinctive spines. Asthenosoma varium is a large sea urchin, hemispherical in shape, and growing to a diameter of 25 cm (10 in). ). The plates from which the test is formed are not joined rigidly together, as is the case in most sea urchin
The Echinothurioida are an order of sea urchins in the class Echinoidea. Echinothurioids are distinguished from other sea urchins by the combination of a flexible test and hollow spines. The membrane around the mouth contains only simple plates, in contrast to the more complex mouth parts of their close relatives, the Diadematoida. They are ...
Sea urchin tests showing the ball parts of the ball and socket joints that articulate with the spines Iconaster longimanus, the icon seastar, showing plate ossicles Ossicles have a variety of forms including flat plates, spines, rods and crosses, and specialised compound structures including pedicellariae and paxillae .
These sea urchins have a disc-like body, more or less bulging, structured by a flexible test, which is nearly unique among sea urchins. Most species can grow quite big and live in deep seas, though some genera contain shallow species (especially Asthenosoma). [1] The test is composed of thin and weakly calcified plates, not always continuous.
These days, long-spined sea urchins are known as the gardeners of the sea. They tend the algae on the coral reefs they call home, making sure it never overwhelms their hosts. Spotting one on the ...
The banded sea urchin has a slightly oval test (shell), reaching a diameter of about 5 cm. [1] Like almost all the Diadematidae (but it is in Echinothrix calamaris that it is most obvious) it has two different sets of spines, short and slender closed spines which go from yellow to dark (through brown) in colour and can deliver a nasty sting, and longer and thicker spines that are often banded ...
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