What are crinoids?
Crinoids are relatively unfamiliar representatives of the Echinodermata, a phylum that also includes sea urchins, starfish, sea cucumbers and brittle stars. At the present day most crinoids belong to a subgroup known colloquially as feather stars and scientifically as comatulids.
At first sight resembling brittlestars, these animals are typically nocturnal inhabitants of coral reefs, clasping onto rocks and corals using their cirri and retreating into crevices during the daytime. In contrast to such mobile crinoids, the deep sea is the home of rare populations of crinoids tethered by stalks to the sea-bed. These are commonly known as sea-lilies, a name alluding to their plant-like appearance.
Sea-lilies have a stem at the top of which is the crown comprising the head and five or more radiating arms used in capturing planktonic food.
Stems may be more than a metre in length and comprise hundreds of segments called columnals. Aggregations of crinoids in the geological past were comonplace, farming 'gardens on the sea-bed.
Crinoid skeletons are made of the readily fossilized mineral calcite. They are the main constituents of some limestones.