septal cycle radial elements

Summary

a set of septa, usually of the same size, inserted (or inferred to have been inserted) within the same corallite, at more or less the same growth stage of the corallite. In rugosans and scleractinians, the first cycle consists of six protosepta. In rugosans, subsequent major septa are progressively inserted into four out of the six proto-interseptal spaces, i.e. those between cardinal septum and the alar pair, and between the counter-lateral and alar pairs. Minor septa are progressively inserted between some or all of the previously inserted major septa. In scleractinians, septa are inserted in apparently simpler cyclical fashion, firstly (S1) in all six of the proto-interseptal spaces, and subsequently (S2, S3,... etc.) in the interseptal spaces between the septa of all the preceding cycles. Hence, in the ideal model of septal insertion in a monocentric corallite, if all cycles are complete, the numbers of septa are hexameral in plan: 6 S1 (total of 6 septa), 6 S2 (12), 12 S3 (24), 24 S4 (48), 48 S5 (96), and so on. However, departures from the ideal plan are quite common. Depending on the taxon, scleractinian cycles are often, but not always, of the same size order, so (e.g.) 5 cycles may occur in only 3 orders. In tabulate corals, regularity of cyclical insertion is not confirmed, but patterns of 12 septa are typical of some groups, while in others, two size orders occur, possibly indicative of cyclical insertion. (Note that septal orders are directly observable whereas cycles can only be inferred from orders, or determined from growth studies.) See also Metaseptum, Protoseptum, Major septum, Minor septum, Septal size order, Septal insertion.

Orthography

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This term is a sub-term of radial elements.

Description

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This version was contributed by Ken Johnson on 22-Apr-2008 14:35.