Ultrastructure and development of forked and capillary setae in the polychaetes Orbinia bioreti and Orbinia latreillii (Annelida: Orbiniidae)

Abstract. Recent investigations into chaetogenesis of certain types of annelid setae provide important results for unravelling the phylogenetic relationships within several taxa of poly-chaetous annelids. This paper presents data on ultrastructure and development of 2 types of orbiniid setae. The analysis of the crenulate capillaries in Orbinia latreillii reveals a formation process which clearly differs from the development of Equisetum -like setae of lingulid bra-chiopods. For the investigation of forked setae, which up to now have been neglected in the discussion on the phylogenetic significance of annelid setae, notopodial setal sacs of O. latreillii and O. bioreti were studied by light- and electron microscopy. In the setal sacs, stages of forked setae are restricted to a dorsocaudal pouch, which represents the site of setal formation. The 2 diverging, stout tines of the fork bear spines on their inner margins, each of which is preformed by a single microvillus. After retraction of the microvilli, a characteristic pattern of the setal canals inside of the spines remains. The present study belongs to a series of comparative studies into chaetogenesis of forked setae. These special setae are also found in other orbiniid taxa as well as some paraonids, scalibregmatids, and nephtyids. Ultrastructural investigations into the development of these forked setae might suggest homology.