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Crinoids, somtimes called ''sea lilies'', are marine animals characterized by an exoskeleton of calcite plates, jointed arms that radiate from the body, and usually by a stem that attaches the animal to a substrate - usually the sea floor. Crinoids are echinoderms and are closely related to sea urchins and starfish. These ''spiny skinned'' organisms first appeared during Early Ordovician times and are still living today. During the Paleozoic Era, they became very abundant. In fact, the Mississippian Period has long been known as the "Age of Crinoids". Vast colonies of crinoids lived in shallow seas during this time, and their remains built up beds of limestone hundreds of feet thick.


Crinoids, like other echinoderms, typically exhibit pentameral (five-fold) symmetry. The calcitic plates that enclose the soft parts is called the calyx. The bottom of the calyx is termed the cup, while the top of the calyx is named the tegmen. The calyx with the arms intact is called the crown. These arms are appendages of the calyx and are generally freely moveable. They are responsible for trapping particles of food and transporting these particles to the mouth by means of a "food groove", located on the inner portion of the arm. The arms of crinoids are quite varied. The most primitive crinoids had only five arms. In many advanced forms, the arms branched numerous times, creating a more effective food gathering mechanism. Once the food has been moved to the mouth by the arms, the crinoid digests these nutrients. Somewhere on the calyx, there is an anal opening to release excrement - sometimes it is a small hole, sometimes it is a long tube. Occasionally, animals such as gastropods and starfish feed at the end of these tubes. The food is not very nutritious, but it is a steady source of nourishment. This symbiotic relationship has existed for millenia - gastropods and starfish both have been fossilized feeding from a crinoid's anal tube. The stem of the crinoid consists of circular, elliptical, or pentagonal plates that are stacked on top of one another to form a column. Articulating surfaces of the individual plates may have permitted some movement of the stem. The stem is perforated by an axial canal which may have transported dissolved lime salts which could be turned into calcite to aid in the growth of the crinoid. The base of the stem may have "root-like" branches, an expanded form of attachment or anchoring device, or it may taper to a point.


Most fossil crinoids had long, jointed stalks, however, present-day crinoids are much more diverse. Many lack stems and have become more mobile - drifting through the ocean currents. Modern crinoids occupy many ecological niches in today's seas. They have been found in shallow water near shoals and reefs and have also been located at depths of more than 13,000 feet. There are more than 6,000 species of fossil crinoids, but there are only 25 stalked genera and 90 free-floating types in today's seas.


Crinoids are classified mainly according to the plate structure of the cup and the make-up of the arms. Because "complete" crinoids are rarely found as fossils, this classification is an ongoing process that will, undoubtedly, be revised as time goes by. But it is easy to see that crinoids were one of the most diverse animals that lived in the ancient seas.

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