This phylum includes three classes: the hydras, Hydrozoa; the jellyfishes, Scyphozoa; and the corals, Anthozoa.  This last class is by far the most important geologically.  The phylum is composed of multicellular, radially symmetrical, mostly marine organisms with differentiated tissues.
 

Class ANTHOZOA

This class includes sea anemones, sea fans, sea pens, sea feathers and corals.  This last group, exclusively marine, ranges from Ordovician to Recent.  Because of their extensive skeleta, corals have an excellent fossil record.  Three orders within this class are particularly important.
 

Order RUGOSA (horn corals)

Corals of this order essentially look like cones with greater or lesser taper.  Septa (internal partitions) radiate from a central axis.  They may be solitary or colonial.
 

Order TABULATA (tabulate corals)

This order derives its name from the diagnostic tabulae, flat plates which provide a floor for the small (less than 1/2 cm) polyp chamber.  This colonial order was an important reef builder of the Paleozoic sea, and in the Great Lakes region these reefs have often accumulated petroleum.
 

Order SCLERATINIA (modern corals)

Triassic to Recent.  They are superficially similar to compound rugose corals.  They can be distinguished from tabulate corals by their lack of tabulae.  Technically they are separated from rugose corals by their sixfold rather than the fourfold symmetry of rugose corals.  Practically, modern corals are only rarely hexagonally appressed (packed like a honeycomb) as are rugose corals and the septae often extend beyond the side wall.

Information provided by: http://inst.santafe.cc.fl.us