Crinoidsare marine animals that make up the classCrinoideaof theechinoderms(phylum Echinodermata). Crinoidea comes from the Greek wordkrinon, "a lily", andeidos, "form".They live both in shallow water and in depths as great as 6,000 meters.[citation needed]Sea liliesrefer to the crinoids which, in their adult form, are attached to the sea bottom by a stalk.Feather starsorcomatulidsrefer to the unstalked forms.
Crinoids are characterized 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 recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to asubstrate, 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 thicklimestonebeds dating to the mid- to late-Paleozoicare almost entirely made up of disarticulated crinoid fragments.
Morphology
A fossil of a typical crinoid, showing (from bottom to top) the stem, calyx, and arms with cirri
Crinoids comprise three basic sections; the stem, thecalyx, and the arms. The stem is composed of highly porous ossicles which are connected by ligamentary tissue. The calyx contains the crinoid's digestive and reproductive organs, and themouthis located at the top of the dorsal cup, while theanusis located peripheral to it. The arms displaypentamerismor pentaradial symmetry and comprise smaller ossicles than the stem and are equipped withcirriwhich facilitate feeding by moving the organic media down the arm and into the mouth.
A stalked crinoid photographed on a wall off New Providence, the Bahamas
The majority of living crinoids are free-swimming and have only avestigialstalk. In those deep-sea species that still retain a stalk, it may reach up to 1 metre (3.3 ft) in length, although it is usually much smaller. The stalk grows out of theaboralsurface, which forms the upper side of the animal in starfish and sea urchins, so that crinoids are effectively upside-down by comparison with most other echinoderms. The base of the stalk consists of a disc-like sucker, which, in some species, has root-like structures that further increase its grip on the underlying surface. The stalk is often lined by small cirri.
Like other echinoderms, crinoids have pentaradial symmetry. The aboral surface of the body is studded with plates of calcium carbonate, forming an endoskeleton similar to that in starfish and sea urchins. These make the calyx somewhat cup-shaped, and there are few, if any, ossicles in the oral (upper) surface. The upper surface, ortegmen, is divided into fiveambulacral areas, including a deep groove from which thetube feetproject, and fiveinterambulacral areasbetween them. The anus, unusually for echinoderms, is found on the same surface as the mouth, at the edge of the tegmen.
The ambulacral grooves extend onto the arms, which thus have tube feet along their inner surfaces. Primitively, crinoids had only five arms, but in most living species these are divided into two, giving ten arms in total. In most living species, especially the free-swimming feather stars, the arms branch several times, producing up to two hundred branches in total. The arms are jointed, and lined by smaller feather-like appendages, orpinnules, which also include tube feet.
