CRINOIDS or "sea-lilies" are members of the subphylum PELMATOZOA in a large group of exclusively marine animals, the ECHINODERMS. Most of the group exhibit a distinct five-fold radial symmetry. Crinoids, for example, have five grooves that radiate out from the mouth at the centre of the cup outward and up the arms to channel food to the mouth. The columnals, which form the stem, often have a central five-pointed star-shaped perforation and tube to hold the stem together and to carry nutrition along the stalk to the roots. The accompanying drawing was compiled from many sources, and shows the main parts of the Crinoid: the cup with arms and the stem with its holdfast. The cup contained the soft, vital organs of the creature. The oldest Crinoids of the Ordovician period were mostly of small size and simple structures, and like most Echinoderms they were gregarious; Paleozoic limestones are frequently called crinoidal limestones. More complicated forms began to appear during the Ordovician period, but they did not have the long stalks of up to fifty feet which some descendants grew. Crinoids are found from the Ordovician period to the present. There are even stemless adult Crinoids among the living representatives.
The CRINOID STEM FRAGMENTS, pictured on the next page, confirm that some variation of shape and form could occur in the parts of the animal. The segments in the top photo have a pentagonal shape, and the lower group of columnals are round. Since the more fragile calyx and roots disintegrate easily, most fossil Crinoids occur as stem fragments when the central tube also collapses after the animal dies.
The SUBPHYLUM PELMATOZOA in the ECHINODERM group also includes cystoids and blastoids. The SUBPHYLUM ELEUTHEROZOA in the ECHINODERM group includes starfishes, "brittle stars", sea urchins, heart urchins, sand dollars, and sea cucumbers.