1.  Five fold symmetry:  arms or rays occurs in multiple of 5's.
2. Advanced Biological systems:
        --Digestive system including mouth, alimentary canal (intestine), and anus
        --Water vascular system: hydralic system which driven by water and muscular contraction serve to motion
             tube feet.  These tube feet, small structures with sucker-like endings, provide for locomotion
             and or feeding.
         -- Reproductive system
         -- Nervous system
         -- Skeleton made of CaCO3 which is developed as discrete plates or segments, each comprising a
               single crystal of calcite.  This monocrystalline structure is particularly illustrated in the structure of
               spines of the urchins.  Here, spines easily cleave along crystal faces after penetrating the soft fleshy

A.  Class Crinoidea -- Sea Lillies
B.  Class Asteroidea -- Starfish
C.  Class Holothuroidea -- Sea Cucumbers
D.  Class Ophuroidea -- Brittle Stars
E.  Class Echinoidea -- Sea Urchins

Role of Echinoderms on Reef:

        Amongst this group of animals, the starfish and urchins are generally the most destructive.  For example the feeding system of the common "regular urchin" (these are lantern shaped with the mouth on the base and anus positioned on the upper part of the structure.  The mouth of the urchins comprises a structure termed Aristole's lantern.  This structure house a set of five regenerative teeth (they are progressively replaced as they wear during normal use) which are manipulated by a set of strong muscles.  Urchins use these teeth to scrape and nibble at the reef (generally dead coral skeleton) in search of minute particles of organic sustenance.  The effectiveness of this feeding structure is evidenced by the volume of material that passes through their digestive systems in a single day.  Scientists have measured quantities of reef particles gouged from the reef skeleton on the order of 2 tablespoons per day (approx. 5 grams).  When on considers the number of urchins that typically occupy the reef, the volume of reef that is destroyed (or recycled) by urchins is a phenomenal quantity of material.  Despite such destructive effects, most urchins only feed upon dead areas of the reef unless their population size expands abnormally above the sustainable limits of the reef.  A common reflection of the presence of urchin is a cavernous, sculpted surface to the reef as these often create small depression or cavities through their incessant bioerosion of the coral skeleton.

A typical black urchin "Diadema" grazing the surface of the reef for particles of algae commonly produces deep cavities and depression on the reef surface.

Perhaps one of the most fascinating of the Echinoderms is the Feather Stars, or the Crinoids.  These are filter feeding animals that use their multiple arms to trap suspended food particles and direct them toward the mouth.  In this case, the mouth is centered at the base of the arms.  Note also, that in these examples, the crinoid is able to attach itself to the surface of the reef with small grasping appendages called Cirri, which allow the crinoid to temporarily hold fast on the surface of corals.
 

    In contrast to urchins, the starfish can active prey upon living coral.  For example, Acanthaster....." the crown of thorns" has nearly devastated large areas of the Pacific reefs.  These starfish feed by extending their stomachs over the living coral tissues.  Secretion of gastral juices (and in the absence of TUMS), progressively digests the living tissue which is then absorbed into the stomach walls of the starfish.  When the feast is completed, the starfish moves on to the next area of living coral polyps.  In the case of the Great Barrier Reef of Australia, the density of these starfish predator was so great that they covered as much as 50 percent of the surface of the reef.  Fortunately, a viral infection ultimately lead to a sharp decline in the numbers of starfish preying upon the reefal systems.  Though saved for the short term, the imbalances in the ecosystem created by mankind will likely manifest in yet another opportunistic expansion of this or another species which may devaste the living reef community.
 
 

These photos illustrate the "Christmas Tree Worm", passive member of the reef community.  In the photo to the left, these worms actively extend their feeding structures to filter particulate matter from the passing currents of water.  If disturbed, they rapidly retreat into their "worm tubes" which are passively constructed alongside the living coral (Right figure).
 
 
 

In general, Mollusca (snails and clams) are relatively passive components of the reef system.  In the case of Snails (Class Gastropoda), feeding is effected through a rasping or scraping of the surface of coral reef.  Yes, snails have teeth.... small scrapers made of CaPO4 (same material of which your teeth are made), and as in the case of the urchins, these are generally passive in their feeding on the reef.  They graze upon the algae which colonize areas of dead coral or of loose debris derived from the the breakdown of the reef skeleton.

In general, most of the clams are passive filter feeders and are passive members of the reef.  However, on group of clams is perhaps on of the more interesting organisms that live with the reef.  These are the boring "rock eating" clams.  These are termed lithophaga (rock eaters). Boring deep within the discarded skeletons of the coral reef, these clams secrete a mild acid (in addition to mechanical abrasion) to bore through the solid rock structure of the reef in search of minor quanities of organic foodstuffs.  Spending their entire life moving progressively deeper and farther into the reef, their burrows or tunnels are distinctive as they expand into conical structures reflecting the increasing growth and size of the clam with age.