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