9/19/2023 0 Comments Do sponges have legs to move![]() ![]() 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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