Crinoids are one of the classes of animals that make up the phylum Echinodermata, which includes about 700 species of marine invertebrates, mainly found in deep waters. Their body resembles a goblet; they have five or more feathery arms, edged with feathery processes which contain their reproductive organs. There are also numerous tube feet placed on them, which perform a sensory function. The arms of the crinoids are equipped with so-called gutters, with tiny, hairy cell processes – the cilia – transporting food to the mouth. Their very characteristic internal skeleton makes thousands of extinct species extremely important Palaeozoic index fossils.