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Abstract
The medusae of many hydroids remain attached to the colony, often in a vestigial state. Evolutionary origins are sometimes thus obscured. Some authorities have considered these derived hydroid life-cycles as paedomorphic; and regarded the hydroid stage as a prolonged larva. In some genera, for example in the familiar but unusual Obelia, a re-evolution of the medusa stage has been proposed.
These theories are evaluated. Primitively, the larval stage throughout the Cnidaria is the planula. Some possible processes in the re-evolution of the medusa are discussed but the hypothesis is discounted. The origin of medusoid coelenterates is taken to be wholly polypoid, medusae being evolved later as modified and released reproductive polyps. The subsequent evolutionary retention and suppression of the medusa stage is discussed and some physiological processes perhaps leading to it are considered. This direction of evolution is related to the advantages of perennation in large, iterated colonies and consequent larval philopatry. Certain confusing terminology is clarified. The term ‘leptolids’ is rescued from disuse, with the original meaning ‘hydroids and/or hydromedusae’ but implicitly including the planula stage as well, to fill a tiresome gap.
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