Larval development and the formation of the gut ofSiboglinum poseidoniFlügel & Langhof (Pogonophora, Perviata). Evidence of protostomian affinity

Abstract

Abstract Larvae of Siboglinum poseidoni were reared in the laboratory. About 6 weeks after recovery from the central Skagerrak the larvae develop a small tentacular bud. On the opposite side a stomodaeum is formed. Electron microscopy of suitable sections shortly in front of the tentacular bud reveals a ciliated opening and tract. In the trunk this tract is reduced to a small tube by bulging endoderm cells containing large amounts of yolk. The lumen is obscured by densely packed cilia. In more advanced stages extracellular symbiotic bacteria occur within this transient digestive tract. They appear to be taken in via the transient mouth. As development progresses the mouth and ciliated foregut close. In the trunk of adult specimens the space between the dorsal and the ventral vessel is occupied by bacteriocytes forming the trophosome. The lumen ofthe digestive tract is reduced to small ciliated spaces. The tentacle develops opposite to the transient mouth and hence dorsally, while the main nerve trunk is ventral. More advanced larval stages closely resemble an elongated annelidian metatrochophora. A relationship between Vestimentifera (Obturata) and Pogonophora (Perviata) is confirmed and the placement of both groups within the phylum Annelida suggested.