ABSTRACT
In tro d u ctio n . The title of this chapter refers to the fact that the groups considered below are of still unknown or uncertain phylogenetic position, and most of them have been placed incertae sedis at various positions in recent phylogenetic analyses (Rouse and Fauchald 1997; Glasby et ah 2000; Rouse and Pleijel 2001). These groups, namely Polygordiidae, Protodrilida, Nerillidae, Dinophilidae, Diurodrilidae, Aeolosomatidae, Potamodrilidae, Parergodrilidae and Hrabeiella periglandulata, mainly comprise meiofaunal or interstitial species. They are characterized by small body dimensions and a seemingly simple organization. Most of the species inhabit marine sediments in intertidal and subtidal regions, but limnic, terrestrial, as well as continental ground water species are also included in this assemblage of polychaetes. Formerly some of these taxa were thought to belong to a single systematic group of more or less primitive annelids, Archiannelida (e.g., Westheide 1990). Because of their simple organization they were considered to represent a primitive state in Annelida, close to the annelid stem species. However, modern investigations have revealed that they neither form a monophyletic group nor retain many plesiomorphies (Hermans 1969; Jouin 1971; Westheide 1985, 1990). Instead these groups have proved to be highly derived taxa, most likely secondarily simplified in certain characters, but highly specialized in others and miniaturized in the course of invading meiofaunal habitats (Jouin 1967, 1968, 1978-79; Westheide and Riser 1983; Westheide 1985; Bunke 1986; Purschke and Jouin 1988; Nordheim 1989b, 1991a, 1991b; Purschke and Jouin-Toulmond 1993; Kristensen and EibyeJacobsen 1995; Purschke and Muller 1996; Rota 1998; Hessling and Purschke 2000; Muller and Westheide 2002; Purschke 1985a, b, 1990a, b, 1992, 1993, 1999, 2002, 2003). Moreover, most of them are obviously not closely related to one another. As a consequence, the concept Archiannelida has been eliminated from modern zoological systems. However, the phylogenetic relationships of most groups remained obscure and unresolved (see e.g.,
Glasby et ah 2001; Rouse and Fauchald 1997), and it is conceivable that some groups may fall into well-known taxa comprising "typical" polychaetes. Although for some of these families no autapomorphies have yet been found (see Fauchald and Rouse 1997), they are usually regarded as being monophyletic.
