Chrysopetalidae Ehlers 1864

Family Chrysopetalidae Ehlers, 1864 Diagnosis (from Watson, 2022). Body length very short to very long; flattened or arched dorsum, flattened ventrum. Prostomium well developed with median antenna (lacking in Calamyzinae), two lateral antennae and differentiated ventral palps or simple lobe-shaped prostomium with undifferentiated two lateral antennae and palps; eyes present or absent. Nuchal organs present in all free-living taxa. First or tentacular segment achaetous, with pair of tentacular cirri in most taxa, variable in symbiotic calamyzins. Midbody chaetal segments with dorsal and ventral cirri. Muscular pharynx with eversible proboscis and pair of grooved stylet jaws; terminal proboscidial papillae present except in symbiont taxa; mouth appendages present in chrysopetalins and dysponetins. Notochaetae paleate, spinose or absent; neurochaetae compound and/or simple; chaetal types camerate. Pygidial lobe with or without pair of anal cirri. Remarks. Bold text indicates characters present in all taxa within the family Chrysopetalidae. Camerate paleal notochaetae was initially proposed as evidence for monophyly of Chrysopetalidae (Westheide & Watson Russell, 1992; Fauchald & Rouse, 1997). Internal cameration of chrysopetalid noto and neurochaetae is seen in all Chrysopetalinae, Dysponetinae and free-living Calamyzinae; lack of neurochaetal compartmentalisation in adults of symbiotic calamyzins is hypothesised as secondary loss (Aguado et al., 2013; Watson et al., 2016). Watson and Faulwetter (2017) considered the grooved jaw form, present across taxa in all chrysopetalid subfamilies, as a synapomorphy that supports the monophyly of the family Chrysopetalidae.