Echinoids, commonly known as sea urchins and sand dollars, are marine animals belonging to the class Echinoidea. They have a hard, spiny exterior called a test, which provides protection and support. Echinoids are found on ocean floors worldwide, from shallow waters to deep sea environments. These creatures play a vital role in marine ecosystems, primarily as herbivores that graze on algae. Their feeding habits help maintain the balance of underwater plant life. Echinoids also serve as important food sources for various predators, including sea otters and fish.