The eleutherozoans are free-swimming, bottom-dwelling, echinoderms which
have been divided into two classes. The class Asterozoa (star-shaped
echinoderms) contains the subclasses Asteroidea (the starfishes) and the
Ophiuroidea (the brittle stars). Although they are known as fossils,
neither of these groups is of paleontological importance. The class
Echinozoa (echinoderms without laterally directed arm-like extensions)
contains the subclasses Echinoidea (the sea urchins and sand dollars)
and Holothuroidea (the sea cucumbers). Of these two subclasses, only the
Echinoidea are useful fossils.