Articulata (Crinoidea)
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 A Passion Flower feather star
(Ptilometra australis) |
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Bourgeticrinida |
Articulata are the only extant subclass of the class Crinoidea. They are differentiated from the extinct subclasses by their lack of an anal plate and the presence of entoneural system. Articulata are stalked echinoderms with pentamerous symmetry. Their stalk, which consists of numerous disks held together by ligaments, supports an aboral cup made of circlets of calcerous plates. five arms, which consist of articulated series of ossicles, extend from the aboral plate and form the food-capture mechanism of articulata. The arms of articulata are pinnulate in that they have alternating pinnules branching out along them to effectively increase the surface area for feeding. These pinnules all have ciliated ambulacral grooves that converge to form larger grooves in the arms that lead down to the mouth located beside the anus on the upper surface of the aboral plate. Articulata are passive suspension feeders. They capture algea with triplets of tube feet located on the pinnules, and the ciliated ambulacral canals transport this algae to the mouth. Although they are passive feeders, some articulata have been observed to move to better feeding areas either with locomotory mechanisms at the base of the stalk or by detaching and pulling themsleves with their arms. Currently there are 540 described species of articulata that fall into two major groups. The bourgueticrinids which have the traditional stalked body form account for about 15% percent of the known species.
[edit] Evolution
Articulata first appeared in the fossil record during the Triassic period although other, now extinct crinoid groups, originated in the Ordivician.
