| 1. | With five pairs of pereopods (thoracomere 2 entirely fused to cephalon, with its appendages modified as pylopods and functioning as a second pair of maxillipeds; thoracomere 8 reduced, without legs); adult males with mandibles grossly enlarged, forceps-like, projecting in front of head; adult females without mandibles. | Gnathiidea |
| - | With seven pairs of pereopods (thoracomere 2 not fused with cephalon, with one pair of maxillipeds and seven pairs of pereopods); males without projecting, forceps-like mandibles; females with mandibles. | 2 |
| 2. | Adults obligate parasites on other crustaceans; bilateral symmetry reduced or lost in females; male a small bilaterally symmetrical symbiont living on the body of the female; antennae (antennae 2) vestigial; antennules (antennae 1) reduced to three or fewer articles; without maxillules (maxillae 1). | Epicaridea |
| - | Not obligate parasites on other crustaceans; bilateral symmetry retained in both sexes; male not as above; antennae never vestigial; antennules variable; usually with maxillules | 3 |
| 3. | Body cylindrical or tubular in cross-section, but often appearing laterally compressed (amphipod-like) due to ventrally elongated abdominal pleura; with distinct row of filter setae along medial margin of maxilla (maxilla 2); penes located on coxae of male pereopod 7; apex of pleotelson curves dorsally; pleonite 5 elongate, markedly longer than any other pleonites (known only from Southern Hemisphere and India). | Phreatoicidea |
| - | Body variable, but not appearing laterally compressed as above; without row of filter setae along medial margin of maxilla; penes on sternum of male pereonite 7 (or on sternum of pleonite 1); apex of pleotelson does not curve dorsally; pleonite 5 rarely elongate (markedly longer than other pleonites only in Limnoriidae) | 4 |
| 4. | Terrestrial; antennules vestigial, minute; pleon always of 5 free pleonites, plus the pleotelson. | Oniscidea |
| - | Aquatic; antennules normal, or if reduced not minute; pleon variable, with or without fused pleonites. | 5 |
| 5. | Anus and articulating base of uropods positioned terminally (or subterminally) on pleotelson; uropods styliform. | 6 |
| - | Anus and articulating base of uropods positioned at base of pleotelson; uropods flattened. | 8 |
| 6. | With lateral coxal plates; antenna peduncle 5-articulate; maxillipeds without coupling setae; penes of male arise from articulation between pereonite 7 and pleonite 1; mandible without palp; pleopodal exopods broad and opercular to the thick tumescent endopods; female pleopod 1 present. | Calabozoidea |
| - | Without lateral coxal plates (pereopodal coxae small); antenna peduncle 6-articulate; maxillipeds with or without coupling setae; penes of male arise on sternum of pereonite 7; mandible with palp; pleopods not as above; female pleopod 1 absent. | 7 |
| 7. | Minute, usually less than 3 mm long; long and slender, length about 6 times width; antenna peduncle without a scale; antennule reduced, peduncle indistinguishable from flagellum; maxilliped without coupling setae on endite; female pleopod 2 biramous; male pleopod 2 endopod not geniculate; interstitial. | Microcerberidea |
| - | Rarely minute, usually greater than 4 mm long; body not elongate (length less than 6 times width); antenna peduncle usually with a scale; antennule rarely reduced, peduncle and flagellum distinct; maxilliped almost always with coupling setae on endite; female pleopod 2 uniramous; male pleopod 2 endopod large and geniculate; rarely interstitial. | Asellota |
| 8. | Body elongate, length usually more than 6 times width; uropodal exopod curving dorsally over pleotelson; coxae of maxillipeds fused to head (i.e. not freely articulating); mandible with lamina dentata in lieu of spine row and lacinia mobilis (lamina dentata, spine row and lacinia mobilis lacking in Paranthuridae); maxillule an elongate stylet with apical hooks or serrate margin; maxilla vestigial and fused with paragnath (or absent). | Anthuridea |
| - | Body not markedly elongate, length usually less than 4 times width; uropodal exopod not curving over pleotelson; coxae of maxillipeds not fused to head; mandible without lamina dentata; maxillule variable; maxilla well developed, never fused with paragnath. | 9 |
| 9. | Uropods modified as a pair of ventral opercula covering the entire pleopodal chamber; males with penes arising on sternum of pleonite 1, or on articulation between pereonite 7 and pleonite 1; mandibular molar process a stout, flattened grinding structure. | Valvifera |
| - | Uropods not modified as ventral opercula covering pleopods, but positioned laterally; males with penes arising on sternum of pereonite 7; mandibular molar process usually a thin, bladelike, cutting structure, or absent (flattened only in Sphaeromatidae). | Flabellifera |
