Echinoids (Class Echinoidea) are spiny marine invertebrates, like sea urchin, sand dollars, and heart urchins , characterized by a hard, fused skeleton called a “test,” five-part symmetry (pentaradial), and tube feet for movement and feeding, living on the seafloor and leaving a rich fossil record. They belong to the phylum Echinodermata, sharing traits with starfish and sea cucumbers, and are divided into regular (sea urchins) and irregular (sand dollars, heart urchins) forms.
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