Sea lily, delicate, deep-sea echinoderm (class Crinoidea) shaped like a plant. It is related to the starfishes. Its body has a skeleton of chalk and consists of a stalk with five arms at one end. The arms branch repeatedly so that the animal has a feathery appearance and is often called the feather star. The largest star lily is 2 ft (61 cm) long, but 70-ft (21-m) fossils have been found. They used to be thought rare, but now they are sometimes dredged up by the ton.