The Class Echinoidea includes sea urchins, sand dollars, and heart urchins—marine invertebrates found in oceans worldwide. They have a hard, spiny exoskeleton called a test, radial symmetry, and a unique Aristotle’s lantern structure for scraping or chewing food. Echinoids are mostly herbivorous or detritivorous, grazing on algae or organic matter on the sea floor. They play important ecological roles by controlling algal growth and recycling nutrients in marine ecosystems.