Crinoid with diver in background (#27A)
Crinoids are the only Echinoderms with their mouth facing upward (in the center of the arms). They are plankton feeders, and catch their food from the passing current. Other Echinoderms such as starfish, brittle stars, and sea urchins are bottom feeders; they have their mouths on the bottom side of their bodies.

This crinoid has taken advantage of an elevated position from the bottom to feed on plankton from the passing current..

Some crinoids may have as few as five arms; others may have up to 200. If an arm is broken off, it is regenerated. As with other Echinoderms, such as Starfish and Brittle Stars, a Crinoid's body is symmetrically divided into five sections; any one section may be completely regenerated if lost.

Biologists theorize that the number of crinoid arms for a particular species has to do with the availability of food-- if the water is rich with plankton, there are fewer arms, since less tentacle surface area is needed to survive. Shallow, warm waters of the ocean reef are less rich in nutrients than deep, cold waters; shallow-water crinoids generally have more arms.

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