The recent Echinodermata are divided in six classes: the Crinoidea (sea lilies and feather stars), the Asteroidea (sea stars), the Concentricycloidea (sea daisies), the Ophiuroidea (brittle stars), the Echinoidea (sea urchins and sand dollars) , and the Holothuroidea (sea cucumbers). Echinodermata are, except for a few brackish water forms, exclusively marine. Adult echinoderms are benthic — a few pelagic Holothuroidea are the exception, however, absent from our area —Êmost of them occur in the intertidal or subtidal, but also deep-sea echinoderms are known.
The basic, adult pattern is pentamerous radially symmetrical, but the larvae, when present, are bilaterally symmetrical. Besides the five-part symmetry, adult echinoderms share two other unique features: an internal calcareous skeleton and distinctively divided coelom including a water vascular system, composed of a complex series of fluid-filled canals, which is usually evident externally as muscular podia, or tube feet. Echinoderms are mostly dioecious and have a direct or indirect development.
Development
The fertilised egg develops into an embryo that displays bilateral symmetry end eventually becomes a free-swimming planktotrophic or lecitotrophic larva. The surface of the larva is beset with ciliated locomotory bands. Lateral development in most echinoderms involve the formation of slender projections (arms or "spines") from the body wall. The nature and position, or even the absence of these arms, distinguishes the different echinoderm classes. For instance, rather short arms or just small exuberances are characteristic for the bipinnaria and brachiolaria larva of Asteroidea [Asteroidea bipinnaria-1 Asteroidea brachiolaria ] and the auricularia and doliolaria larva of Holothuroidea. Long, stiff, spine-like arms (supported by an internal skeletal structure) are characteristic for the pluteus larvae of the Class Echinoidea [Echinoidea pluteus ] and the Class Ophiuroidea [Ophioruidea pluteus]. The larval arms disappear in later development; they are not synonymous with the arms of some adult echinoderms (as, for instance sea stars and brittle stars).
After spending its time as a plankter, the larva attaches itself to a bottom substrate to prepare for the metamorphosis. During this transformation, the bilateral symmetry makes place for a radial symmetry and most larval structures are lost, finally the juvenile assumes a benthic life.
[Mostly after Brusca and Brusca, 1990]
The following echinoderm taxa figure in the key:
Phylum Echinodermata
Class Echinoidea
Class Ophiuroidea
Class Asteroidea
Order Paxillosida
Family Luidiidae
Luidia ciliaris
Luidia sarsi
Order Forcipulatida
Family Asteriidae
Asterias rubens