The illustrations (collected from different sources mentioned in the text), the key and the diagnoses in this document were prepared for students in the course Nematode Taxonomy and Systematics. Other users are welcome.

2 (1)- Corpus greatly enlarged, stoma long slender, tooth
          absent ..........................................................CYLINDROCORPOROIDEA
             Only one family...................................................... Cylindrocorporidae ..3

        Corpus strongly enlarge but longer, stoma wide, short
        with tooth and denticles ...................................ODONTOPHARYNGOIDEA
           One family.....................................................................Odontopharyngidae
               Only one genus  (Fig. 1) ..................................................Odontopharynx

3 (2)- Gonads paired...............................................................................................4
          Gonad single..................................................................................................5

4 (3)- Bursa rudimentary, leptoderan or absent (Fig. 2)........................Myctolaimus
          Bursa easily seen, peloderan  (Fig. 3) .....................................Myctolaimellus

5 (3)- Male tail long, bursa leptoderan (Fig. 4)....................... Protocylindrocorpus
          Male tail short, bursa peloderan  (Fig. 5) ........................................ Goodeyus

DIPLOGASTEROIDEA

6 (1)- Stoma with 2 U-shaped structures followed by narrow tube;
          two knob-like structures present at end of tube..................... Tylopharyngidae
                     Only one genus  (Fig. 6) ...................................................Tylopharynx

         Stoma not as above.........................................................................................7

7 (6)- Both median bulb and basal bulb muscular; median bulb with weak crescentic
          valvular thickenings; basal bulb with valvular apparatus
          ......................................................................... Pseudodiplogasteroididae 8
          Basal bulb never muscular, never with valve ...................................................9

8 (7)- Gonads paired  (Fig. 7) ...........................................Pseudodiplogasteroides
          Gonad single  (Fig. 8) ................................................Protodiplogasteroides

9 (7)- Stoma oblong with  small ridge-like denticles on metarhabdion
          ...................................................................................Diplogasteroididae 10
          Stoma wide, rarely oblong, usually with teeth, denticles or ridges on metarhabdion
          ........................................................................................Diplogasteridae 15

10 (9)- Amphids large at mid-stoma level
            .................................................................................Diplogasteroidinae 11
           Amphids small at the base of lateral lips.........................Rhabditolaiminae 13

11 (10)- Female gonad single  (Fig. 9) ..........................................Diplogasteroides
             Female gonads paired ..............................................................................12

12 (11)- Cheilorhabdion short, not cuticularized (Fig. 10)........................... Goffartia
              Cheilorhabdion longer, cuticularized  (Fig. 11) ...................Paramonovnema

13 (10)- Gonad single  (Fig. 12) ......................................................Rhabditolaimus
              Gonads paired...........................................................................................14

14 (13)- Stoma long, 5 times as long as wide, female tail
              conical  (Fig. 13) ...............................................................Dirhabdilaimus

             Stoma shorter, about 3 times as long as wide, female
             tail subulate or filiform  (Fig. 14) ......................................Rhabdontolaimus

15 (9)- Body asymmetrical, left side with wards, right side with longitudinal ridges
            ...............................................................................Heteropleuronematinae
                   Only one genus  (Fig. 15) ........................................Heteropleuronema
           Body symmetrical, both sides similar ............................................................16

16 (15)- Median bulb elongate, dorsal metarhabdion with a
              broad median ridge..........................................................Demaniellinae 17

             Median bulb not elongate; metarhabdion with tooth,
             teeth or denticles...........................................................Diplogasterinae 18

17 (16)- Lip edges thickened; female gonads paired (Fig. 16) .............Demaniella
             Lip edges not thickened; female gonad single  (Fig. 17) ...Metadiplogaster

18 (16)- Gonad single ...........................................................................................19
              Gonads paired ........................................................................................20

19 (18)- Cheilostom about twice as long as protostom (Fig. 18).......Monobutlerius
              Cheilostom as long as or shorter than protostom  (Fig. 19) .........Acrosticus

20 (18)- Cheilorhabdion divided into 18 ribs; metarhabdion with 2 large teeth; telostom
              very long (4 times as long as wide) with 2 wing-like structures at its base
             (Fig. 20)..... ..................................................................... Neodiplogaster
             Cheilorhabdion not so; number of teeth variable, telostom much shorter........21

21 (20)- Spermatheca (uterus) kidney-shaped..........................................................22
              Spermatheca (uterus) not kidney-shaped....................................................23

22 (21)- Female with stomatal dimorphism, tail conical, spicules with strong hook at
              the end  (Fig. 21) .............................................................Aduncospiculum

              Female without stomatal dimorphism, tail long and fine spicules never with a hook
              (Fig. 22)...........................................................................Diplogasteritus

23 (21)- Female tail very short, less than 2 anal body widths  (Fig. 23) .. Diplenteron
              Female tail longer...................................................................................... 24

24 (23)- Dorsal metarhabdion with claw-like tooth ..................................................25
             Dorsal metarhabdion with hump-like (protruding), pyramidal, thorn-like tooth
             or with ridges .............................................................................................33

25 (24)- Stoma 2 forms: stenostoma similar to Paroigolaimella but narrower; eurystoma
              similar to Mononchoides; right subventral metarhabdion with a tooth or plate,the
              left one with dentate structure; telostom veryshort  (Fig. 24) .... Pristionchus
             Stoma normal, not as above........................................................................26

26 (25)- Right-subventral metarhabdion also with a claw-like tooth (in addition to the
             dorsal claw-like tooth)................................................................................27
             No claw-like tooth on left or right subventral metarhabdion (in addition to the
            dorsal claw-like tooth).................................................................................31

27 (26)- Lip ring with 12-18 ribs, each rib bifid at its anterior end ...........................28
              Lip ring without bifid ribs...........................................................................29

28 (27)- Telostom almost as long as protostom (Fig. 25) .................Mononchoides
              Telostom very short, about one third of protostom  (Fig. 26) ............ Fictor

29 (27)- Lip ring deeply grooved by incisions; each groove with a single anterior
              process....................................................................................................30

              Lip ring without grooves, telostom cylindrical or funnel-shaped with two knob-
              like teeth at base  (Fig. 27) ....................................................... Koerneria

30 (29)- Stoma as long as wide, telostom very short  (Fig. 28) ...........Oigolaimella
              Stoma longer than wide, lip ring with 6 grooves,
              telostom long  (Fig. 29) .........................................................Micoletzkya

31 (26)- Lip ring as in Mononchoides, claw-like tooth raised, subventral metarhabdion
              smooth  (Fig. 30)  .......................................................Eudiplogasterium
              Stoma not as above................................................................................32

32 (31)- Spicules fused  (Fig. 31) ................................................Pareudiplogaster
             Spicules not fused  (Fig. 32) ....................................................Glauxinema

33 (24)- Stoma with 6 teeth, 2 subdorsal and 4 subventral, meso, meta and telorhabdion
              fused  (Fig. 33) .......................................................................Gerthornus
              Number of teeth less than 6; meso, meta and telorhabdion not fused.........34

34 (33)- Dorsal metarhabdion pyramidiform with 2 inclined ridges, male with large bursa
              (Fig. 34)  .......................................................................Diplogasteriana
              Dorsal metarhabdion not pyramidiform, male may have rudimentary bursa..35

35 (34)- Stoma wide, lip region with 12 grooves, dorsal metarhabdion with a thorn-like
              tooth, subventral metarhabdion with rasp or wart plates  (Fig. 35)
              ........................................................................................ Paroigolaimella
              Stoma not as above..................................................................................36

36 (35)- Dorsal metarhabdion with a humped tooth; subventral metarhabdion with
              two small humps in tandem  (Fig. 36) ...............................Allodiplogaster
              Dorsal metarhabdion with large or small tooth or teeth, subventral
              metarhabdion without two small humps in tandem.....................................37

             Dorsal metarhabdion with two hook-like teeth or the whole metarhabdion with
             3 almost the same size, triangular teeth..................................................... 40

37 (36)- Cheilostom longer than promesostom......................................................38
              Cheilostom equal to or shorter than promesostom...................................39

38 (37)- Esophagus close to Odontopharynx but with a valve in swollen median bulb;
              one dorsal and one smaller subventral tooth present on metarhabdion; head
              with 6 prominent setose papillae  (Fig. 37) ..................................Butlerius

             Esophagus typical diplogasteroid; dorsal metarhabdion with a large pyramidal
             tooth, subventral metarhabdion with a small sclerotized tooth; head with small
             papillae  (Fig. 38) ................................................................... Diplogaster

39 (37)- Cheilostom as long as promesostom; dorsal metarhabdion with a small tooth;
              spicules fused, tail spicate  (Fig. 39) ...........................................Masseyus

              Cheilostom shorter than promesostom; dorsal tooth large; spicules not fused,
              tail filiform, very long  (Fig. 40)  ..............................Mesodiplogasteroides

40 (35)- All three teeth in the stoma large, triangular, almost equal in size; cheilostom
              and promesostom almost equal  (Fig. 41) ......................Anchidiplogaster
              Dorsal metarhabdion with 2 hook-like teeth, left subventral segment with axe-
              shaped tooth, right subventral with 2 small pyramidal teeth; cheilorhabdion
              and promesorhabdion fused to form an  inverted funnel (Fig. 42)
              ...................................................................................... Paradoxogaster