Sea urchins
Sea urchins are a class of marine animals that live on the seabed or burrow into the sand. Those that live on rocky seabeds are spherical and have prominent, protective spines. Those that live on, or burrowed into, soft sand are heart-shaped or flattened, like the sand dollar, and have short spines that make them look furry.
Order
Echinoida
Purple sea urchin (species)
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About
Sea urchins or urchins are small, spiny, globular animals that compose part of class Echinoidea. They are found in all oceans. Their shell, or "test", is round and spiny, typically from 3 to 10 centimetres (1.2 to 3.9 in) across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae. Sea otters, wolf eels, and other predators feed on urchins. Humans harvest them and serve their roe as a delicacy.
Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms they have fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The symmetry is not visually obvious, but is easily visible in the dried test.
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 the oral opening, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, and brittle stars.
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Elsewhere on the web
- Sea urchin larvae (microscopy-uk.org.uk)
- Sea urchin biology (nhm.ac.uk)
- Sea urchin (wikipedia.org)
Site links
BBC links
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