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Crinoids belong to echidermates with more or less developed calyx, stem and arms. The shape of the calyx is variable – conical, hemi-conical, oval, etc. It is composed from 2‒3 rows of calcic plates and grows branched arms of different length. The stem is assembled from flat segments (of the circular-like, aster-like, square-like shape at the cross-section) in a way enabling flexion. Crinoids are exclusively marine animals, their habitat is benthic or planktonic. First Crinoids occurred in the Ordovician.
## Keywords
multicellular animals, Phylum Echinodermata, class Crinoidea, Ordovician - Recent
