1	Thick simple, Y-shaped chaetae in mid body (enlargement and fusion of shafts and blades)	Syllis gracilis Grube, 1840 (1)
–	Without these thick simple chaetae	2
2	Aciculae of posterior parapodia distally rounded and hollow. Pharyngeal tooth distinctly back from the pharyngeal opening	3
–	Aciculae not as above. Pharyngeal tooth located on anterior margin	4
3	Compound chaetae distinctly bidentate, with both teeth similar	Syllis prolifera Krohn, 1852 (1)
–	Compound chaetae with unidentate blades or with minute, spine-like proximal tooth	Syllis vivipara Krohn, 1869 (1)
4	With spiniger-like compound chaetae	5
–	Without spiniger-like compound chaetae	12
5	Aciculae of posterior parapodia thick, straight, acute, protruding from the parapodial lobes	6
–	Aciculae otherwise	7
6	Mid body dorsal cirri elongated. Mid body spiniger-like chaetae distinctly bidentate	Syllis cornuta Rathke, 1843 (11)
–	Mid body dorsal cirri fusiform. Mid body spiniger-like chaetae indistinctly bidentate	Syllis mercedesae Lucas, San Martín & Parapar, 2012 (10)
7	Proximal tooth of spiniger-like chaetae and falcigers distinct, forming a narrow angle with distal teeth (both teeth almost parallel); apparently without eyes	Syllis caeca (Katzmann, 1973) (11)
–	Chaetae not as above; eyes present	8
8	Mid body dorsal cirri thick, short and fusiform	Syllis parapari San Martín & López, 2000 (1)
–	Dorsal cirri slender, more or less elongated	9
9	Posterior aciculae distally bent at an angle. Dorsal simple chaetae truncate. Short spiniger-like chaetae, distally rounded and unidentate from mid body	Syllis rosea (Langerhans, 1879) (1, 11)
–	Aciculae acuminate. Dorsal simple chaetae acute. Arrangement and shape of spiniger-like chaetae not as above	10
10	Spiniger-like chaetae very short, only present on anterior and mid body segments; spiniger-like chaetae and falcigers unidentate, sometimes with a long, slender subdistal spine	Syllis oerstedi (Malmgren, 1867)
–	Chaetae not as above	11
11	Blades of falcigers with long spines on margin, especially distally, extending beyond level of proximal tooth	Syllis garciai (Campoy, 1982) (1)
–	Spines of blades not so long, decreasing distally, not reaching level of proximal tooth	Syllis mauretanica (Licher, 1999) (11)
12	On mid body, one thick simple chaeta on each parapodium, formed by blade loss and shaft enlargement	Syllis amica Quatrefages, 1866 (1)
–	Without thickened, simple chaetae	13
13	Posterior aciculae distally bent at an angle. Dorsal simple chaetae truncate	14
–	Without the above characters	15
14	Proventriculus long, through about 5 segments or more. Two dorsal glands after proventriculus	Syllis pulvinata (Langerhans, 1881) (1)
–	Short proventriculus, through 3 segments. Dorsal glands absent	Syllis gerlachi (Hartmann-Schröder, 1960) (1)
15	Dorsal cirri of mid body short, fusiform	14
–	All dorsal cirri elongated, not fusiform	16
14	Dorsal cirri strongly fusiform. Mid body compound chaetae almost unidentate, with a short, small proximal tooth	Syllis armillaris (O.F. Müller, 1771) (1, 11)
–	Dorsal cirri not so strongly fusiform. Mid body compound chaetae bidentate	Syllis hyalina Grube, 1863 (1, 11)
16	Aciculae of posterior parapodia thick, straight, acute, protruding from the parapodial lobes	17
–	Aciculae otherwise	20
17	Blades of compound chaetae unidentate (or slightly bidentate on anterior parapodia)	18
–	Blades distinctly bidentate	19
18	Dorsal cirri long. Blades distally more or less hooked	Syllis fasciata (Malmgren, 1867) (11)
–	Dorsal cirri short, slender, delicate. Blades short, triangular	Syllis licheri Ravara, San Martín & Moreira, 2004 (1)
19	Dorsal cirri short, slender, delicate. Posterior aciculae distally bent, oblique, although pointed. Without colour pattern	Syllis pontxioi San Martín & López, 2000 (1)
–	Dorsal cirri longer. Aciculae straight. Strong pigmentation on anterior segments, as ∞	Syllis variegata Grube, 1860 (1, 11)
20	Compound chaetae all unidentate, distally acute	Syllis vittata Grube, 1840 (1, 11)
–	At least anterior compound chaetae bidentate	21
21	Long dorsal cirri of anterior segments distinctly thicker than others. Compound chaetae of posterior segments distinctly enlarged, unidentate or with a small proximal tooth. Anterior segments pigmented with distinct transverse red bands	Syllis krohnii Ehlers, 1864 (1, 11)
–	Dorsal cirri of similar thickness throughout body. Pigment pattern otherwise	22
22	Posterior compound chaetae unidentate by reduction and loss of distal tooth. Prostomium, peristomium and chaetiger 1 with dark red pigment, sometimes also a small red band on some anterior segments	Syllis torquata Marion & Bobretzky, 1875 (1)
–	Without such colour pattern nor such chaetae	23
23	Compound chaetae strongly bidentate. Colour pattern: one rhomboidal red mark on dorsum and a slight line on each border of each segment	Syllis columbretensis (Campoy, 1982) (1)
–	Compound chaetae slightly bidentate Colour pattern forming ∞ on anterior segments	Syllis westheidei (San Martín, 1982) (1, 11)