Crinoidea is a class of marine animals commonly known as sea lilies or feather stars. They belong to the phylum Echinodermata, which also includes starfish and sea urchins. Crinoids have a cup-shaped body called a calyx, with feathery arms that extend outward to capture food from the water. These creatures are primarily found in deep-sea environments, although some species inhabit shallow waters. Crinoids are filter feeders, using their arms to trap plankton and small particles. They have a long evolutionary history, dating back to the Cambrian period, making them one of the oldest living groups of animals.