External gestation in exogonine syllids (Annelida : Polychaeta): dorsal egg attachment by means of epitokous chaetae

Abstract

Sexually mature individuals of three exogonine syllid species carry eggs or embryos on the dorsolateral body surface above the parapodia. They are attached to the body by chaetae that either penetrate the egg envelope (Sphaerosyllis hermaphrodita) or are closely apposed to the eggs (Brania sp. and B. subterranea). The chaetae are delicate capillaries packed together in bundles; they originate in epidermal thickenings or sacs situated dorsally and slightly anterior to the dorsal cirri. These chaetae are present only in sexually mature animals, and thus are interpreted as epitokous structures, but they are not homologous with the epitokous swimming chaetae situated below the dorsal cirri occurring in several syllids.