None. Begins: "Protodrilidae is a family of interstitial annelids lacking parapodia, chaetae and appendages except for two anterior palps. Their body ranges in length from 1 to 13 mm and comprises a rounded prostomium, a comparatively large peristomium, a cylindrical trunk with epidermal cilia and glands, and a pygidium with two adhesive lobes (Fig. 1A–C) (von Nordheim 1989a, Westheide 2008). The most conspicuous external characters of the family are their paired, filiform, ciliated palps arising anteroventrally at the prostomium provided with an internal coelomic channel connected behind the brain and their locomotion by ventral ciliary gliding. Protodrilidae currently consists of 36 described species, which are now grouped into six genera corresponding to monophyletic clades of a recent phylogenetic study employing molecular and morphological data (Martínez et al. 2015). These genera are Protodrilus Hatschek, 1881, being the type genus, Meiodrilus Martínez, Di Domenico, Rouse and Worsaae, 2015, Megadrilus Martínez, Di Domenico, Rouse and Worsaae, 2015, Lindrilus Martínez, Di Domenico, Rouse and Worsaae, 2015, Claudrilus Martínez, Di Domenico, Rouse and Worsaae, 2015 and Astomus Jouin, 1979. This last genus has been previously referred to as Parenterodrilus (see remarks below) (Jouin 1992).
Early workers included Protodrilidae within the now disregarded group Archiannelida. Some of them interpreted the simple morphology of Protodrilidae as primitive and close to the annelid stem species (Czerniavsky 1881, Hatschek 1881, Pierantoni 1908), whereas others suggested that this apparent simplicity was rather the result of a secondary adaptation to interstitial life (Goodrich 1901, Remane 1932, Hermans 1969). More recent workers regarded Archiannelida as polyphyletic and placed Protodrilidae, Protodriloidae and Saccocirridae in the taxon Protodrilida mainly based on the resemblance of the pharynx and palps among the three families (Purschke & Jouin 1988). This placement was supported by cladistic morphological analyses by Rouse and Fauchald (1997) and Parry et al. (2016), in which these three families were nested within the large clade Canalipalpata, together with Spionida, Terebellida and Sabellida. Despite this morphological support, Protodrilida was never recovered in any molecular analysis using single or a few genes (Rousset et al. 2007, Struck et al. 2007, Zrzavý et al. 2009), rising doubts on its monophyly. However, two recent phylogenomic analyses independently recovered Protodrilidae in a clade with Protodriloididae, Saccocirridae and Polygordiidae, consistently as first branch within Errantia (Andrade et al. 2015, Struck et al. 2015). With several large-sized annelid taxa branching off at deeper nodes than Protodrilidae (e.g., Weigert et al. 2014, Andrade et al. 2015, Struck et al. 2015, Parry et al. 2016) all phylogenetic studies support a secondary reduction in size and form, which in combination with the lack of larval-juvenile traits suggest that the slender elongate body and simple morphology is the result of gradual miniaturization rather than of progenesis (Westheide 1984, 1987, Worsaae & Kristensen 2005, Struck et al. 2015).
