Sunday, October 21, 2007

Animalpedia: Criniods

This is a picture of a crinoid. Crinoids are in the same phylum as sea urchins, cucumbers, and sea stars, the echinoderms (spiny-skin). Basically, if you imagine an upside-down brittle star with lots of arms, you’ve got a crinoid. There are two types, stalked crinoids, which have a stalk giving them the common name of ‘sea lilies’, and stalk-less crinoids, which have claw like cirri that they use to walk with.

Both types are filter feeders, and you often find them in deeper waters where they catch ‘marine snow’ detritus and dead matter that falls from the upper surface. Their tube feet are modified to assist in filter feeding, they lack the suction cups on the end that most other members of the echinoderm phyla posses. Also, they are one of the few members of the echinoderms whose anus is on the same side as their mouth, instead of the opposite side. (The other group that has the same arrangement is the sand dollars)

They first appeared in the oceans around 510 million years ago, around the middle of the Cambrian beginning of the Ordovician periods. They were incredibly abundant from the Silurian to the Carboniferous periods (435-290 mya), such that whole layers of ocean limestone are made up entirely of their fossils.

While modern stalked crinoids only reach a height of 60cm (23 inches), some fossil forms reached 20 meters in height (60 feet)!

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