1	Nuchal organs as two occipital lappets. Pharynx more or less sinuous and coiled	2
–	Nuchal organs as two ciliated pits (difficult to observe; occipital lappets absent or, if present, with transversal ridges) between prostomium and peristomium. Pharynx straight	4
2	Body composed of few, rhomboidal segments. Ventral cirri well developed. Last segment without chaetae, with two pairs of long cirri	Amblyosyllis
–	Body composed of numerous, cylindrical segments. Ventral cirri absent or fused to parapodial lobes. All segments (except peristomium) chaetigerous	3
3	Two antennae. Ventral cirri distinct, fused along ventral side of parapodial lobes. Compound chaetae with long, filiform, unidentate blades. Reproduction unknown	Acritagasyllis
–	Three antennae. Ventral cirri apparently absent (totally fused with parapodial lobes?). Compound chaetae with short blades, usually with proximal tooth longer than distal. Reproduction by epigamy or schizogamy	Subfamily Autolytinae
4	Pharynx unarmed	5
–	Pharynx with mid-dorsal tooth, trepan or both	6
5	Palps fused along their entire length. Antennae and tentacular cirri minute, papilliform. Single pair of tentacular cirri. Reproduction by epigamy	Anguillosyllis
–	Palps fused basally. Antennae and tentacular cirri more or less club-shaped. Two pairs of tentacular cirri. Reproduction by epigamy or brooding eggs ventrally	Subfamily Anoplosyllinae
6	Antennae, tentacular cirri and dorsal cirri distinctly articulated, usually long (two genera with only one spherical article). Reproduction by schizogamy (some viviparous)	Subfamily Syllinae
–	Appendages smooth or weakly articulated on anterior part of body. Reproduction by epigamy (but unknown in several genera)	7
7	Palps fused entirely or at least to mid way along their length. Antennae, tentacular cirri and dorsal cirri short (sometimes papilliform). Eggs brooded dorsally on capillary notochaetae, or ventrally, attached to nephridial pores	Subfamily Exogoninae
–	Palps not completely fused. Appendages long, filiform. No brooding of eggs; reproduction by epigamy (or unknown)	Subfamily Eusyllinae (plus some incertae sedis genera)