STL Science Center

STL Science Center

13 April 2017

Forgotten but Popular

Crinoids possess a fairly simple appearance from an external view. A large stalk or stem, a calyx, and a feathery appendage are used for stability and locomotion, feeding and reproducing, and directing food into the mouth respectively. We can think of Crinoids as upside down starfish with the stalk growing out of what would be the dorsal surface of the starfish and a sucker or root-like tendrils anchoring the animal when it does not want to float on the currents. Some of these root structures are comprised almost entirely of the cirri that originate in the stalk. Their tough fibrous nature allows certain Crinoids to use the cirri as small and mostly inefficient legs as well, moving the Crinoid slowly from area to area. It is this stalk that is so often discovered by amateur fossil hunters in roadcuts in places like Missouri and Kansas; in case you find the shear number of fossils hard to believe consider these links please: (Missouri state fossil and Crinoid Stonehenge Model). The calyx is also frequently found, complete with the feathery arms, but is still not as regularly discovered as the stalk by amateurs. The calyx has, like other echinoderms, mouth, reproductive organs, and anus in close proximity to one another; if this sounds confusing, consult this graphic to the right. The feathery arms of the animal are called Pinnules and are ciliated, or covered in small hair-like structures called cilia that are capable of moving gathered food items toward the oral cavity of the animal. Crinoids add even more amazingness to their life history in that they are pentaradial echinoderms. Echinoderms are unique in that they begin life as bilaterally symmetric ("mirrored") larvae and during their life cycle grow to be pentaradial, or having five main segments arranged at 72ยบ intervals around the mouth. Consider the ontogenetic image below to better visualize this change.

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