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Corals
Corals (together with other animals
such as jellyfish) belong to the phylum Coelenterata (meaning hollow gut). Only some Coelenterata secrete a hard skeleton, of which the most common is the coral.
Important groups include the tabulate corals (Tabulata), an extinct group found throughout the Palaeozoic Era. These were colonial corals made of many interlinked tubes called corallites. The corallites
are braced with horizontal sheets called tabulae. Favosites (the honeycomb coral) is a common genus from the Silurian Period.
Solitary corals are much larger reef builders; they dominated the corals found in Upper Palaeozoic times, such as the Early Carboniferous Period. It is, therefore, called the Age of Corals.
Solitary corals begin from a basal plate, and the coral polyp then secretes
its skeleton on top of this in the form of an outer wall and inner radial dividers called septa. An axial column occupies the centre of the structure. In section, these parts are easily identified (see opposite). There are two groups, the rugose corals (Rugosa), or wrinkled corals, which belong entirely to the Palaeozoic Era, and hexacorals, which are Mesozoic Era to recent. The rugose corals have four major septa, and the hexacorals have six.
Coral reefs and coral rocks do not consist entirely of corals, and corals do not always live in reefs, as you can see in the samples here.
Mouth in centre
Tentacles
Septa
Soft polyp, which is not preserved, lives inside structure below.
Tabula
Axial column or columella
Thecal wall
Dissepiments
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Coral exoskeleton or corallite
(Above) A coral polyp, showing hard and soft parts.
(Above) A fossil corallite.