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. Zero one, or two pairs of eyes, a series of paired accessory eyes sometimes present along body. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Three or four pairs of eyes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Eyes close together (separated by at least the diameter
Five pairs of eyes forming a regular arch; jaws typically present and toothed, but may be toothless or absent; one to several pairs of large gastric caeca; most species highly pigmented; medium to very larg size, to 22 mm in length. . . . 8
Three or 4 pairs of eyes (rarely absent) never arrranged in a regular arch; jaws absent; gastric caeca absent; body linear; pigmented or not; occasionally over 100 mm in length but usally much less, 20-50 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erpobdelliada, Erpobdella
3. Body terete, pale, translucent. . . . . . . . . . . . . . . . . . . . . . Gloiobdella sp.*
Body wide and flattened, pigmented; about 5 species, most common and widespread genus of glossiphoniids. . . . . . . . . . . . . . . . . . . . . . Helobdella
4. Eyes in parallel rows; pigment with 2 distinct paramedial dark stripes; adults about 20 mm; parasitic and predacious, chiefly on mollusks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glossiphonia complanata
Eyes in weak triangular pattern, middle pair farthest apart; witout pigment; adults less than 10 mm. . . . . . . . . . . . . . Alboglossiphonia heteroclita
5. Caudal sucker flattened, as wide or wider than the widest part of body, often with ocelli; vesicles on margin of trunk; suckers distinctly set off from body; 2 pairs of eyes or none. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Caudal sucker concave, weakly developed, and narrower than widest part of body, without occelli; vesicles asent; 1 pair of eyes. . . . . . . . . . . . . . . . . . . 7
6. Vesicles small, difficult to see on preserved leeches; body weakly or not divided into anterior and posterior parts. . . . . . . . . . . . . . . . . . . . . . . . .
Vesicles large, obvious after preservation; body strongly divided into anterior and posterior parts. . . . . . . . . . . . . . . . . . . . . . . Cystobranchus verrilli
7. Body slim; colorless or scattered pigment cells; 24-26 mm long. Myzobdella
Body thicker; with 6 longitudinal rows of brown-black pigments; 6-8 mm long. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Piscicolaria*
8. Ventral surface with gonopores only; large species, often mottled, very dark or with a median stripe. . . . . . . . . . . . . . . . . . . . . . Haemopidae, Haemopis
Ventral surface with either porous, grandular region around gonopores or patch of grandular pores posterior to gonopores; usually with bright-colored markings. . . . . . . . . . . . . . . . . .. Macrobdellidae, Macrobdella decora