In this paper a description of Acritagasyllis longichaetosus, an enigmatic genus and species of Syllidae (Polychaeta), is given. The new genus is characterized by a lack of palps, eyes, and median antenna, having a single pair of tentacular (peristomial) cirri, distinct ventral cirri fused all along their length to parapodial lobes, a slender, coiled pharynx without trepan but with a pharyngeal tooth, compound chaetae with long shafts, distally enlarged and spinose, and long, filiform blades, quite similar to those belonging to members of the family Phyllodocidae. These characters are very unusual and unique among syllids. Some characters, like the presence of nuchal lappets, bayonet-shaped simple chaetae, and perhaps also the fusion of ventral cirri to parapodial lobes, bear some resemblance to those of species of Autolytinae. However, this new polychaete does not fit into this subfamily as it possesses ventral cirri, the chaetae are very different, and it lacks a trepan in the pharynx, nor can it be placed into any other subfamily, being an “incertae sedis” among the genera of Syllidae.

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