A crinoid is a marine animal of the class Crinoidea. There is only one extant subclass of crinoids, the Articulata, consisting of 540 described species, though other subclasses once existed but are now extinct. Crinoids, also called sea-lilies or feather-stars, are feathery or spiny invertebrates consisting of a number of arms around a central, top mouth. They may be fixed to a substrate or free-swimming, and some types of crinoid take both forms at different stages of the life cycle.
Crinoid species are quite diverse, though not nearly as plentiful as they once were. Extinct crinoid species are known from fossils from the Paleozoic era. Today, crinoids may live in very shallow waters or at depths of up to four miles (six kilometers). Crinoids also vary widely in appearance, though many are colorful and reminiscent of flowers.
A crinoid consists of three basic body parts: the calyx, the arms, and the stem. The calyx consists of the digestive and reproductive systems and is surrounded by the arms, or brachials. The brachials typically follow a five-fold symmetry and are covered with thin pinnules, in turn covered with cilia, to increase surface area and help move food toward the center mouth. The stem extends downward from the calyx, opposite from the brachials. Most modern crinoid species, about 85%, lack a stem.
Modern crinoids are the last remaining remnant not only of the class Crinoidea, but of a much more extensive population of filter-feeding echinoderms. Throughout the Paleozoic and Permian eras, Crinoidea had competition from such filter-feeders as blastoids, edrioasteroids, and others. A major extinction event occurring at the boundary between the Permian and Triassic eras, about 251 million years ago, saw the disappearance of about 95% of the world's marine life, including 98% of crinoid species and 100% of other filter-feeding echinoderms. Articulata developed somewhat later than other crinoid species, first appearing in the fossil record during the Triassic period.