Hard corals comprise the Subclass Zoantharia (Hexacorals).  These are distinct from the Subclass Octacorallia (soft corals) based on several characteristics.

HARD CORALS:
        1. Produce a rigid skeleton comprising CaCO3.
        2. Structural elements of this skeleton:
               Tabulae -- horizontal partitions which allow for upward growth of polyp by isolating the surficial
                "living skin" from the discarded skeleton.
               Septae -- Vertical plates which serve to support the internal folds (mesenteries) in the gastrovascular cavity.
        3.  Septae, Tentacles and Mesenteries are emplaced in sets of 6. Thus the term, hexacorals.
        4.  Tentacles are smooth without additional pinnate fringes.
        5.  This group typically possesses zooxanthellae symbionts which aid in nutrition and fromation of the skeleton
                of the coral.  When symbionts are present, we term these hermatypic corals.
 

The radial array of tentacles about the central oral disk (mouth) of a stony coral are emplaced in sets of 6.  These are relatively simple tentacles in structure without fringes oramented only by small protruding cells representing the cnidoblasts.  Note that this is a colonial coral comprising several individual polyps.  Each of these polyps are genetically identical, having been produced through a process of asexual reproduction (budding).  In the above figure, one can also observe the sutures between individual polyps.  This represents a continuous layer of tissue or living skin that drapes across the surface of the underlying hard skeletal structure that builds the reef.  This connection of tissue provides for the distribution of nutrients as well as communication amongst the individual of this large colony.

SOFT CORALS
        1. Produce a flexible skeleton that is variably mineralized.  This skeleton can be composed of sclerites
            (small fragments of skeleton) composed of either CaCO3 or Gorgonin.
        2. Mesenteries and Tentacles are emplaced in sets of 8.  Thus, octocorals.
        3. Tentacles are complex with small pinnate fringes.
        4. Soft corals typically are ahermatypic, do not possess symbionts.

This example of a seafan illustrates the fringed and 8-fold character of the tentacles of the soft corals (Subclass Octacorallia).  Note that these fans are also colonial corals comprising numerous individual polyps.
 
 

    Distributed within the tissues of many corals are a group of photosynthetic yellow-brown algae (dinoflagellates) termed zooxanthellae symbionts.  This mutual relationship of benefit, or symbiosis, is a major factor in the success of recent coral reefs in developing large communites in shallow shelf regions of the marine realm.  Specifically, these reefs develop in oligotrophic (nutrient-poor) settings where recycling of nutrients is an integral part of the success of the reef.  Zooxanthellae through photosynthesis recycle respiratory byproducts from the coral (nitrates and phosphates) and recombine these into carbohydrates that can then be utilized by the coral polyp.  The process of photosynthesis also aids in the formation of the CaCO3 coral skeleton.  In effect, when CO2 is added to water, it produces an acid which dissolves the skeletons of corals.  In contrast, as CO2 is removed from water via photosynthesis by the zooxanthellae, this helps in the formation or precipitation of CaCO3.

    The benefit for the zooxanthellae derives from their residence within the tissues of the coral.  This provides for protection from predation by grazers while simultaneously positioning the algae within the photic zone as the coral progressively grows into shallower waters.

    Interestingly, these symbionts represent an infestation/infection of the coral tissue as it matures.  When conceived in sterile environments, juvenile coral polyps do not possess these symbionts.  Also, there are factors which can lead to the expulsion of the symbionts from the coral tissue.  For example,  during the last El Nino', vast expanses of the coral reefs of the Pacific and Atlantic underwent a process called bleaching due to a rise in surface water temperature.  The long term effects of such bleaching are unknown, however, the short term effects must result in a decrease in nutrient recycling and a overall decline in reef productivity.