![]() Several sea urchins, however, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. The mouth is at the base of the animal and the anus at the top the lower surface is described as "oral" and the upper surface as "aboral". This is most apparent in the "regular" sea urchins, which have roughly spherical bodies with five equally sized parts radiating out from their central axes. Like other echinoderms, sea urchin early larvae have bilateral symmetry, but they develop five-fold symmetry as they mature. The name is derived from the Old French herichun, from Latin ericius ('hedgehog'). The name urchin is an old word for hedgehog, which sea urchins resemble they have archaically been called sea hedgehogs. They have a rigid, usually spherical body bearing moveable spines, which give the class the name Echinoidea (from the Greek ἐχῖνος ekhinos 'spine'). Urchins typically range in size from 3 to 10 cm (1 to 4 in), but the largest species can reach up to 36 cm (14 in). Phyllacanthus imperialis, a cidaroid sea urchin ( Cidaroidea)ĭescription Sea urchin anatomy based on Arbacia sp. 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. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. ![]() Together with sea cucumbers ( Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays. Irregular echinoids include: flattened sand dollars, sea biscuits, and heart urchins. The "irregular" sea urchins are an infra-class inside the Euechinoidea, called Irregularia, and include Atelostomata and Neognathostomata. Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, with 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 them. The symmetry is not obvious in the living animal, but is easily visible in the dried test. Like other echinoderms, they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive " tube feet". Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, sand dollars, brittle stars, and crinoids. Fossil urchins have been used as protective amulets. Species such as the slate pencil urchin are popular in aquaria, where they are useful for controlling algae. That has continued with studies of their genomes because of their unusual fivefold symmetry and relationship to chordates. The animals have been studied since the 19th century as model organisms in developmental biology, as their embryos were easy to observe. ( Sand dollars are a separate order in the sea urchin class Echinoidea.) The closest echinoderm relatives of the sea urchin are the sea cucumbers (Holothuroidea), which like them are deuterostomes, a clade that includes the chordates. The fossil record of the Echinoids dates from the Ordovician period, some 450 million years ago. Like all echinoderms, adult sea urchins have fivefold symmetry, but their pluteus larvae feature bilateral (mirror) symmetry, indicating that the sea urchin belongs to the Bilateria group of animal phyla, which also comprises the chordates and the arthropods, the annelids and the molluscs, and are found in every ocean and in every climate, from the tropics to the polar regions, and inhabit marine benthic (sea bed) habitats, from rocky shores to hadal zone depths. Predators that eat sea urchins include a wide variety of fish, starfish, crabs, marine mammals, and humans. Although algae are the primary diet, sea urchins also eat slow-moving ( sessile) animals. Sea urchins move slowly, crawling with tube feet, and also propel themselves with their spines. Most urchin spines range in length from 3 to 10 cm (1 to 4 in), with outliers such as the black sea urchin possessing spines as long as 30 cm (12 in). The spherical, hard shells ( tests) of sea urchins are round and covered in spines. About 950 species of sea urchin are distributed on the seabeds of every ocean and inhabit every depth zone from the intertidal seashore down to 5,000 meters (16,000 ft 2,700 fathoms). Sea urchins ( / ˈ ɜːr tʃ ɪ n z/) are spiny, globular echinoderms in the class Echinoidea. Tripneustes ventricosus and Echinometra viridis
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