1a	Mouth ventral and posterior to brain; proboscis without modified middle region bearing armature	“Class” Anopla 2
1b	Mouth antero-terminal or subterminal and usually sharing a common opening with the rhynchopore but rarely the two open separately; proboscis with modified middle region bearing armature of one or more stylets	“Class” Enopla 6
2a	Cephalic lobe (head) demarcated posteriorly with pair lateral vertical furrows (may be indistinct)	3
2b	Cephalic lobe lateral margins each a distinct longitudinal furrow	Lineidae 4
3a	Ground color pale ochre; numerous irregularly spaced blackish-brown rings encircle body – most anterior interrupted by mouth, fourth bears pale, often indistinct, oval lateral sense organ	Tubulanus rhabdotus
3b	Ground color pale, milky tan; numerous reddish brown short but irregular length longitudinal lines cover dorsal surface and may extend ventrally; cephalic furrows with orthogonal secondary furrows	Baseodiscus delineatus
4a	Caudal cirrus present in adults but may be very small	5
4b	Caudal cirrus lacking in adults; blackish-brown ground color with numerous pairs of white markings dorsally along length of body; anterior 2/3 of cephalic lobe white with symmetrical brown patterning	Lineus stigmatus
5a	Body uniformly blackish except for white anterior margin of cephalic lobe and white caudal cirrus; mouth longer than relaxed width of worm	Dushia atra
5b	Body uniform bright orange anteriorly, grading to yellowish posteriorly	Micrura ignea
6a	Proboscis armature a fig with numerous tiny tack-like stylets; also numerous sacs of accessory stylets; long and wide oblique cephalic furrows with orthogonal secondary furrows; numerous conspicuous ocelli set in approximately 4 rows	Punnettia cf. natans
6b	Proboscis armature a distinct single stylet resting on a basis, usually with a pair of sacs containing accessory stylets	7
7a	Ocelli present (not always evident)	9
7b	Ocelli absent; statocyst present	Ototyphlonemertes 8
8a	Statocyst with usually 3 statolith granules; stylet smooth; papillae at anterior of anterior proboscis chamber each with rod-shaped inclusion	Ototyphlonemertes erneba
8b	Statocyst with about 12 statolith granules; proboscis extremely short (about length of head); stylet helically sculpted; cerebral organs and cephalic cirri absent	Ototyphlonemertes lactea
9a	Four ocelli set as rectangle	Unidentified monostiliferan spp 1–3 (see text)
9b	More than four ocelli (not always evident)	10
10a	Blood vessels conspicuously filled with red corpuscles	Amphiporus cruentatus
10b	Blood vessels “colorless”	11
11a	Body uniformly dark brown and opaque, cephalic lobe rimmed by white “halo”; about 6 ocelli along each antero-lateral margin visible only in specimens squeezed under coverslip	Amphiporus texanus
11b	Ocelli visible without squeezing specimen	12
12a	Ocelli extend laterally next to and posterior to cerebral ganglia; armature basis concave or flat posterior margin; epidermis contains small, intracellular, crescent-shaped hooks (requires squeezing specimen under coverslip and compound microscopy at 200–400×)	Zygonemertes 13
12b	Ocelli all precerebral; armature basis convex posterior margin	14
13a	Body pale to dark green as adult, may be grayish-white as juvenile	Zygonemertes virescens
13b	Body uniformly rosy pink	Zygonemertes fragariae
14a	Ocelli small, 6-10 in a pair of rows parallel to and near lateral margins of head; body uniformly translucent milky gray to yellowish, and cerebral ganglia with orange hue	Amphiporus cf. ochraceus
14b	Ocelli large, about 26 set in four groups but may appear to form one large group along each lateral margin of the head; body relatively opaque rosy red; armature basis very rounded and short relative to stylet; good swimmer when irritated	Cratenemertidae sp. 1