Echinoidea, commonly known as sea urchins, are marine animals characterized by their spiny exterior and spherical shape. They belong to the phylum Echinodermata, which also includes starfish and sea cucumbers. Sea urchins inhabit various ocean environments, from shallow coastal waters to deep-sea habitats, and play a crucial role in marine ecosystems. These creatures have a hard shell called a test, covered with movable spines that provide protection and aid in locomotion. Sea urchins are primarily herbivorous, feeding on algae and other plant materials, and are important for maintaining the balance of their habitats.