Subphylum Asterozoa

(Ordovician-Recent)

starfish and their relatives

Starfish and brittle stars, as they are popularly called, are members of the group called organisms called the Stelleroidea or Asterozoa which are characterized by a flexible star shaped body which has a central disk with five or more radiating arms. Each arm contains a radial branch of the water vascular system, which supports numerous paired tube feet which extrude through the underside of each arm, alongside the ambulacral groove. (feeding groove). The mouth is located on the underside of the disk, in the center where the radiating ambulacra intersect.

Complete fossil starfish are usually quite rare. Most fossil starfish consist of scattered individual plates or segments of arms. This is because the skeleton is not ridged, as in the case of echinoids (sea urchins), but is composed of many small plates (or ossicles) which quickly fall apart and are scattered after death and the decay of the soft parts of the creature.

Asteroidea - true starfish

Ophiuroidea - brittle stars.