FUNCTIONAL MORPHOLOGY AND DEVELOPMENT OF SEGMENTAL INVERSION IN SABELLID POLYCHAETES

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Abstract
Form and function are analyzed in the sabellid Branchiomma nigromaculata with reference to the ability of such worms to survive radical mutilation by predators. The crucial structural modification in the intact worm is the parapodial inversion of all segments posterior to the first 8 anterior segments. This enables the parapodia to dig into the inner wall of the tube so that a posterior part is inevitably left behind to reconstitute a complete individual. During postlarval growth, in several species, e.g., Potamilla neglecta, Sabella microphthalma and Fabricia sabella, all with comparatively large eggs, all thoracic segments (typically 8) are formed before any abdominal segments are produced. In B. nigromaculata, Potamethus elongatus, and a dwarf species, all with comparatively smaller eggs, less than the full complement of thoracic segments are initially produced, and the remainder are subsequently formed by successive conversion of adjoining abdominal segments.

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