From Wikipedia, the free encyclopedia
Crinoids are marine animals that make up the class Crinoidea of the echinoderms(phylum Echinodermata). The name comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 9,000 meters (30,000 ft). Those crinoids which, in their adult form, are attached to the sea bottom by a stalk are commonly called sea lilies. The unstalked forms are called feather stars[ or comatulids.
Crinoids are characterised by a mouth on the top surface that is surrounded by feeding arms. They have a U-shaped gut, and their anus is located next to the mouth. Although the basic echinoderm pattern of fivefold symmetry can be recognised, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults.
There are only about 600 extant crinoid species, but they were much more abundant and diverse in the past. Some thick limestone beds dating to the mid- to late-Paleozoicare almost entirely made up of disarticulated crinoid fragments.
Crinoids feed by filtering small particles of food from the sea water with their feather like arms. The tube feet are covered with a sticky mucus that traps any food that floats past. Once they have caught a particle of food, the tube feet can flick it into the ambulacral groove, where the cilia are able to propel the stream of mucus towards the mouth. Generally speaking, crinoids living in environments with relatively little plankton have longer and more highly branched arms than those living in rich environments.
The mouth descends into a short oesophagus. There is no true stomach, so the oesophagus connects directly to the intestine, which runs in a single loop right around the inside of the calyx. The intestine often includes numerous diverticulae, some of which may be long or branched. The end of the intestine opens into a short muscular rectum. This ascends towards the anus, which projects from a small conical protuberance at the edge of the tegmen
- Fossilised crinoid columnal segments extracted from limestone quarried on Lindisfarne, or found washed up along the foreshore, were threaded into necklaces or rosaries, and became known as St. Cuthbert's beads.
- In the Midwestern United States, fossilized segments of columnal crinoids are sometimes known as Indian beads.
- Crinoids are the state fossil of Missouri.