The Crinoidea includes the most exquisite members of the Echinodermata, far more stunning as a rule than any plodding urchin or commonplace starfish (just to let you know at the outset where my allegiance lies). With a family tree rooted in almost 500 million years of history, they are the senior group of living echinoderms. Their typically echinoderm features include:

·        5-sided adult symmetry derived from a bilateral larva.

·        Water vascular system - a network of coelomic canals and reservoirs that may serve in respiration, circulation, feeding and locomotion, and that terminates in external podia, or tube feet.

·        Calcareous endoskeleton consisting of individual plates (ossicles) with a meshwork fine structure (stereom), each formed from a single high-magnesium calcite crystal.

·        Mutable collagenous (or catch connective) tissue that can alter between rigid and flaccid states under neuronal control.

·        Deuterostome, enterocoelous embryonic development with radial cleavage (Brusca and Brusca 1990).

 

Features that distinguish crinoids from other echinoderms are:

·        Two to four circlets of ossicles (never 4 in living species) fused together as a cup- or box-like calyx that contains or supports the viscera.

·        Five flexible, usually branched and featherlike rays—extensions of the body wall supported by skeletal plates and bearing food-collecting grooves and extensions of the water-vascular, nervous, haemal and reproductive systems.

·        The oral surface contains both mouth and anus and orients away from the substrate.

 

 

Most of the several thousand known fossil crinoids and several dozen modern deep-water species bear a stalk on the side of the body opposite the mouth (aboral) and are known as sea lilies. Imagine a feathery starfish on a stick. (Thanks, Conrad.) Modern stalked crinoids live almost entirely in waters deeper than 200 m, although Metacrinus rotundus occurs in 100 m off Japan (Oji 1986). Most living crinoids, however, shed their stalk at an early growth stage. Called feather stars or comatulids, they retain only the topmost stalk segment, which usually bears numerous hooks for anchoring the animal. While stalked crinoids have a fossil record dating to the Early Ordovician (Guensburg & Sprinkle 2003), comatulids first appeared during the Jurassic and have radiated substantially since (Meyer and Macurda 1977). They exhibit a morphological plasticity that generates major taxonomic difficulties and suggests that their diversification continues. Comatulids occur throughout modern seas (except the Black and Baltic) from the intertidal zone to abyssal depths. Currently, about 540 nominal species exist in 142 genera and 18 families. By contrast, ~95 stalked species have been described (Roux et al. 2002).
 
This website outlines our current understanding of living crinoids: their phylogeny, distribution and ecology. It also includes a detailed introduction to morphological features, terms and symbology, a discussion of practical aspects of working with specimens and the difficulties associated with species identifications. There's also an artificial key to the families of living crinoids—artificial because detailed phylogenies have not yet been worked out (watch this space).

 

REFERENCES

Ausich, W.I. 1996. Echinodermata. Pp. 242-261, IN: Feldmann, R. M. (ed.) Fossils of Ohio. Ohio Division of Geological Survey, Bulletin 70.

Ausich, W.I. 1997. Calyx plate homologies and early evolutionary history of the Crinoidea. Pp. 289-304, IN: Waters, J. A. & Maples, C. G. (eds.) Geobiology of Echinoderms. Paleontological Society Papers 3.

Brusca, R. & Brusca, G.J. 1990. Invertebrates. Sinauer, Sunderland MA. 922 pp.

Guensburg, T.E. & Sprinkle, J. 2003. The oldest known crinoids (Early Ordovician, Utah) and a new crinoid plate homology system. Bulletin of American Paleontology no. 364:1-43.

Heinzeller, T. & Welsch, U. 1994. Crinoidea. Pp. 9-148 IN: Harrison, F.W. & Chia, F.-S. (eds.) Microscopic Anatomy of Invertebrates. Vol. 14. Echinodermata. Wiley-Liss, New York.

Hess, H., Ausich, W.I., Brett, C.E. & Simms, M.J. 1999. Fossil Crinoids. Cambridge Univ. Press, NY.

Messing, C.G. 1997. Living Comatulids. Pp. 3-30 IN: Waters, J.A. & Maples, C.G. (eds.) Geobiology of Echinoderms. Paleontological Society Papers 3.

Messing, C.G. and Dearborn, J.H. 1990. Marine Flora and Fauna of the Northeastern United States, Echinodermata: Crinoidea. NOAA Technical Report NMFS 91.

Meyer, D.L. and Macurda, D.B., Jr. 1977. Adaptive radiation of the comatulid crinoids. Paleobiology 3:74-82.

Oji, T. 1986. Skeletal variation related to arm regeneration in Metacrinus and Saracrinus, Recent stalked crinoids. Lethaia 19:355-360.

Roux, M., Messing, C.G. & Améziane, N. 2002. Artificial keys to the genera of living stalked crinoids (Echinodermata). Bulletin of Marine Science 70(3):799-830.