The two planktonic larval periods ofLanice conchilega(Pallas, 1766) annelida polychaeta, a peculiar example of the irreversibility of evolution

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Abstract
It is generally assumed that a planktonic larval stage characterizes the life cycle of ancestors of the majority of present-day marine invertebrates. Later, some families, including the Terebellidae, saw the planktonic larval stage considerably shortened or suppressed. Still later, a new planktonic stage that is clearly marked by numerous segments was acquired in the Terebellidae by the genera Lanice and Loimia when life cycles of other species of the same family (e.g. Eupolymnia nebulosa) stayed unchanged. This planktonic stage, the aulophore, follows an intermediate benthic stage which itself follows a short-lived trochophore and metatrochophore. This aulophore stage, probably a recent acquisition, corresponds to a relatively late morphological stage which is benthic in other species of the family. Consequences of the second pelagic phase are examined. This new feeding planktonic stage is not a return to the first pattern of trochophore-metatrochophore series, but represents a new developmental type. Evolution appears reversible when attention is focused entirely on the end products but not when processes are taken into account. Planktonic stages are possibly reached by different evolutionary routes. This idea is based on comparison of the well known life cycles of Lanice conchilega and Eupolymnia nebulosa.