coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

s, a costa (plural costae) is one of the vertical plates lying outside the

corallite A corallite is the skeletal cup, formed by an individual stony coral polyp, in which the polyp sits and into which it can retract. The cup is composed of aragonite, a crystalline form of calcium carbonate, and is secreted by the polyp. Corallit ...

wall, a continuation of a

septum In biology, a septum (Latin language, Latin for ''something that encloses''; septa) is a wall, dividing a Body cavity, cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Hum ...

(plural septa) which lies inside the wall. The costae may continue to the edge of the colony and in solitary species, such as those in the family Fungiidae, refers to the ridges on the underside of the coral.

References

Cnidarian anatomy {{cnidarian-anatomy-stub