Illustrated here are the larval and adult forms of starfish class Asteroidea, phlyum Echinodermata. These particular individuals were photographed off Lizard Island on the Great Barrier Reef, Australia.

The Echinodermata are exclusively marine organisms and are widely distributed upon the benthos, from the intertidal zone to the deep sea. 'Echinodermata' means 'spiny skin', refering to their characteristically tough, spiny exterior (see above right). The phlya comprises five extant classes: the Crinoidea (feather stars & sea lilies), the Asteroidea (starfish), the Echinoidea (sea urchins & sand dollars), the Holothuroidea (sea cucumbers) and the Ophiuroidea (brittle stars). They number about 6000 living species and have a fossil record extending back over 500 million years.

Echinoderms display a five-part radial symmetry (see above centre), which seems to serve their sedentary benthic lifestyle well. They possess an internal skeleton of calcium carbonate plates, called ossicles, and a unique water vascular system. This vascular system consists of a series of canals which radiate throughout the body and terminate in structures called tube feet. These tube feet penetrate the body wall and often have a tiny suction or adhesive cup at their tip. Body fluid, with an ionic composition similar to that of seawater, circulates through the system for hydraulic expansion or contraction of the tube feet. These feet have many functions, including adhesion, locomotion, feeding and respiration.

The majority of echinoderms have seperate sexes, but hermaphrodites occur among asteroids, holothurians and ophiuroids. Many species have external fertilization which produce vast numbers of planktonic larvae (see above left) that consist of a number of distinct stages which often bear little resemblance to the adult form.