1. Mouth a small pore on ventral surface of anterior sucker through which a muscular pharyngeal proboscis can be protuded; no jaws or teeth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Order: Rhynchobdellida, 2
Mouth large, occupying the entire cavity of anterior sucker; no protrusible proboscis; jaws with teeth either present ofr absent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Order: Arhynchobdellida, 3
2. Body flattened dorsoventrally and much wider than head (except Placobdella monifera ); body not cylindrical (except Helobdella elongata, which is subcylindrical); not differentiated into two body regions; anterior (oral) sucker ventral, more or less fused to body and narrower than body; body never divided into anterior trachelosome and posterior urosome; eggs in membranous cocoons and young brooded on ventral surface of parent; 1-4 pairs of eyes; not oculiform eyespots on posterior sucker; segments 3-annulate (a1, a2, a3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glossiphoniidae
Body cylindrical and usually long and narrow; body sometimes divided into a narrow anterior trachelosome and wide posterior urosome (Myzobdella); anterior oral sucker expanded and distinct from body; 0-2 pairs of eyes; pulsate vesicles along the lateral margins present (Piscicola and 6-30 mm Cystobranchus) or absent; seven or more annuli per segment (except Myzobdella reducta, which is a 3-annulate); ovuliform eyespots sometimes present on posterior (caudal) sucker; no brooding of cocoons or young; parasitic on fishes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Piscicolidae
3. Five pairs of eyes arranged in an ocular arch; body elongate; jaws with denticles (teeth) either present or absent; nine or ten pairs of testisacs arranged metametically; pharynx short. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Zero, three, or four pairs of eyes in separate labial and buccal groups; body elongate; no jaws (agnathus). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. One or two crop caeca per somite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Crop acecate except for postcaeca; five pairs of eyespots arranged in an arch on segments II-VI with the third and fourth pairs of eyespots separated by one annulus; gonopores separated by 5-7 annuli; jaws absent or present (distichodont, 9-25 poirs of teeth per jaw); dorsum uniform color with or without black longitudinal middorsal stripe; uniformly dark olive green with faint longitudinal striped along midline and numerous small irregular scattered black flecks; or without dorsal stripes but with few moderately to extensively blotched, spotted or mottled with olive, yellow, dark gray or black. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Haemopidae, Haemopis
5. Ventral surface with glandular area and external area and exgternal copulatory gland depressioons or pores, in some species arranged rows, either surroundeding and directly posterior to female gonopore or ten or eleven annuli posterior to male gonopore; lacking penis sheath; gonopores separated by 2-5 1/2 annuli; monostichodont except for single partially disticbodont species, 20- 65 teeth per jaw; crop with 2 paris of caeca per somite, equal in size; vagina fusiform, vaginal duct short; length 40-150 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Macrobdellidae, Macrobdella decora
6. Ventral surface with a pair of accessory pits (gastropores?), one anterior and one posterior of the gonopores, respecgtively (predaceous). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Salifidae, Barbronia weberi
Ventral surface without accessory pits (gastropores?). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erpobdelliada, Erpobdella