CRINOID

any marine invertebrate of the class Crinoidea (phylum Echinodermata) usually possessing a somewhat cup-shaped body and five or more feathery arms. The arms are edged with feathery projections (pinnules) that contain the reproductive organs and carry numerous tube feet with sensory functions. The tentacles also have open grooves, along which cilia (minute, hairlike projections) sweep food particles toward the mouth. The distinctive limy tests (internal skeletons) of crinoids make the thousands of extinct species (together with extinct echinoderms of similar form) important Paleozoic index fossils. About 700 living species are known, mainly from deep waters. For more information about living crinoid species and groups, see feather star; sea lily.