The phylum Echinodermata has been divided into two subphlya based on the possession of a stem, the Pelmatozoa and the lack of a stem, the Eleutherozoa. The pelmatozoans can be divided into the non-criniods (or blastozoans) and the crinoids themselves. Crinoids, however, are the only surviving pelmatozoans.
J.S. Miller first recognised crinoids as a distinct class in 1821, basing his classification on the structure of the calyx. The subsequent divisions of the crinoids is based on a number of characters such as the structure of the arms, the number of plate circlets and their morphology and the structure of the oral surface.
There have been a number of different classification schemes for the Crinoidea (see Ubaghs, 1978 and Simms & Sevastopulo, 1993), but generally they can be grouped into four main subclasses, the Flexibilia, the Inadunata, the Camerata and the Articulata.
A Phylogenetic Tree of the major families of Crinoidea. Reproduced with permission from Charles Messing.
The Flexibilia, Inadunata and Camerata are all restricted to the Palaeozoic. The effect of the end-Permian mass extinction was to leave only one lineage of crinoids, which developed into the monophyletic subclass Articulata and remain the only extant class. Over 250 genera are known within the Articulata, roughly two-thirds of which are still found today.