1	Dorsal cirri on chaetiger 2 present	2
–	Dorsal cirri on chaetiger 2 absent	4
2	Papillae on dorsum absent	Sphaerosyllis claparedei Ehlers, 1864
–	Papillae on dorsum present	3
3	Parapodial glands absent	Sphaerosyllis papillifera Naville, 1933
–	Parapodial glands with fibrillar material	Sphaerosyllis ovigera Langerhans, 1879
4	Parapodial glands present	5
–	Parapodial glands absent	15
5	Parapodial glands with fibrillar material	6
–	Parapodial glands with granular material	12
6	All antennae in line	Sphaerosyllis capensis Day, 1953
–	Median antenna inserted more posteriorly than lateral ones	7
7	Dorsal cirri shorter than parapodial lobes, at least in anterior chaetigers	8
–	Dorsal cirri longer than parapodial lobes	9
8	Blades of falcigers strongly serrated, short (<10µm); shafts with strong spines	Sphaerosyllis thomasi San Martín, 1984
–	Blades of falcigers with serration only anteriorly and dorsalmost; blades with slight dorso-ventral gradation but always longer than 10µm; shafts smooth	Sphaerosyllis parabulbosa San Martín and López, 2002
9	Blades of falcigers without marked dorso-ventral gradation in length	Sphaerosyllis taylori San Martín, 1984
–	Blades of dorsalmost falcigers at least 1.5 times the length of ventral ones	10
10	Blades of posterior dorsal compound falcigers smooth to finely serrated	Sphaerosyllis hystrix Claparède, 1863
–	Blades of posterior dorsal compound falcigers strongly serrated (spinules of almost same length as the subdistal spine)	11
11	Blades of anterior dorsal compound falcigers at least twice as long as ventral ones; anteroposterior gradation of blade length; blades of both dorsal and ventral compound chaetae with a subdistal spine	Sphaerosyllis boeroi Musco, Çinar and Giangrande, 2005
–	Blades of anterior dorsal compound falcigers less than twice as long as ventral ones; no anteroposterior gradation of blade length; blades of only dorsal compound chaetae with a subdistal spine	Sphaerosyllis levantina sp. n.
12	Blades of dorsalmost falcigers long (>30µm), at least twice as long as ventral ones	Sphaerosyllis magnidentata Perkins, 1981
–	Blades of falcigers short (<15µm), with only slight dorso-ventral gradation	13
13	Dorsal cirri clearly longer than parapodial lobes	Sphaerosyllis sp. [San Martín, 2003]
–	Dorsal cirri as long as or shorter than parapodial lobe	14
14	Antennae bulbous with small tip, shorter than prostomium; dorsal simple chaetae smooth; anterior parapodia with two aciculae, one straight, one with tip bent at right angle	Sphaerosyllis sp. Del-Pilar-Ruso & San Martín, in press
–	Antennae pyriform, as long as prostomium, dorsal simple chaetae serrated, all parapodia with one acicula	Sphaerosyllis glandulata Perkins, 1981
15	All aciculae straight	16
–	Tip of some aciculae bent at right angle	17
16	Dorsal cirri with conspicious papilla, giving cirri a bifid appearance	Sphaerosyllis gravinae Somaschini and San Martín, 1994
–	Doral cirri without papilla	Sphaerosyllis bulbosa Southern, 1914
17	All antennae in line	Sphaerosyllis austriaca Banse, 1959
–	Median antenna inserted more posteriorly than lateral ones	18
18	Anterior parapodia with two aciculae, one straight, one with tip bent at right angle; pharyngeal glands on chaetiger 1 present	Sphaerosyllis pirifera Claparède, 1868
–	All parapodia with one acicula only; pharyngeal glands on chaetiger 1 absent	Sphaerosyllis piriferopsis Perkins, 1981