Where the septa cross the corallite wall they become costae (singular is costa). The costae run between corallites or out towards the edge of the colony. The costae are often distinctive and may be used to differentiate between genera. It is important to be aware that in free-living corals such as Fungia, the costae are considered to be the structures on the underside of the coral.
Septa are vertical skeletal elements inside the corallite wall, and are contiguous with costae when they cross over the wall. Costae may run across the coenosteum between corallites and join with the septa of the next corallite - the genus Favia.
The corallite is the skeleton home built by the coral polyp. At its simplest the corallite structure can be described using three terms - wall, septa and costae. Recognising septa and costae (singular: septum and costa) in all their forms is critical to coral identification.