Crinoids are animals shaped like lilies that eat with their mouths facing up
A crinoid looks like a flower on a stalk, and its name comes from the Greek word for lily. It is really an echinoderm, a cousin of starfish and sea urchins, but turned the other way up: both its mouth and its anus sit on its upper surface, ringed by feathery arms.
The stalked kinds are called sea lilies, while the unstalked feather stars form the biggest order, Comatulida. Most feather stars carry a stem only as juveniles and swim free as adults, often keeping a few root-like cirri at the base. In the deep-sea species that still have one, the stalk can reach a metre, and fossils show stems of 20 metres. Crinoids live everywhere from shallow water to depths beyond 9,000 metres.
Five-part symmetry, the echinoderm signature, is still visible, but the five original arms usually split into 10, and in many feather stars they fork repeatedly into as many as 200 branches. Side branches called pinnules give the feathery look. Three sizes of tube feet line the arms, none with suckers: the longest stand erect to form a sticky trap, and the smaller ones handle what gets caught.
Feeding is passive. A crinoid raises its arms into a fan across the current, and mucus-coated tube feet snag plankton and flick it into grooves where tiny hairs carry it to the mouth. Mobile species climb rocks or coral to catch more flow. The trapping surface can be enormous: one Japanese sea lily's 56 arms, each 24 centimetres long, add up to 80 metres once the pinnules are counted. Where plankton is scarce, arms tend to be longer and more branched.
Only about 700 living species are known, a fraction of their former abundance. Their skeletons are mostly hard plates that fossilise readily, and some thick limestone beds from the mid-Paleozoic to the Jurassic consist almost entirely of crinoid fragments, including Lower Carboniferous rock around Clitheroe in England.
Source: Crinoid
