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Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, including 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 it.
The irregular sea urchins are an infra-class inside the Euechinoidea, called Irregularia, and include Atelostomata and Neognathostomata.
"Irregular" echinoids include: flattened sand dollars, sea biscuits, and heart urchins.
Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays.
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, brittle stars, and crinoids.
but they develop five-fold symmetry as they mature.
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.
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.
In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet.
Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes.