What shape is a sea urchin?
Sea urchin 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 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. Urchins typically range in size from , although the largest species can reach up to . They have a rigid, usually spherical body bearing moveable spines, which gives the class the name Echinoidea (from the Greek "ekhinos", spine). The name "urchin" is an old word for hedgehog,
Sea urchin skeletogenesis Sea urchin skeletogenesis Skeletogenesis is a key morphogenetic event in the embryonic development of vertebrates and is of equal, although transient, importance in the development of the sea urchin, a marine invertebrate. The larval sea urchin does not resemble its adult form, because the sea urchin is an indirect developer, meaning its larva form must undergo metamorphosis to form the juvenile adult. Here, the focus is on skeletogenesis in the sea urchin species "Strongylocentrotus purpuratus", as this species has been most thoroughly studied and characterized. Skeletogenesis begins in the early sea urchin blastula (9–10 hours post fertilization) when the primary