Analysis of information sources in references of the Wikipedia article "Annelid" in English language version.
This is the case for circular muscles, which have been reported as absent in many families (Opheliidae, Protodrilidae, Spionidae, Oweniidae, Aphroditidae, Acoetidae, Polynoidae, Sigalionidae, Phyllodocidae, Nephtyidae, Pisionidae, and Nerillidae; Tzetlin et al. 2002). Tzetlin and Filippova (2005) suggest that absence of circular muscles is possibly plesiomorphic in the Annelida.