1- Stoma narrow; esophagus without median bulb; head with separate lips, labial edges
    strongly cuticularized or head with cirrated appendages anteriorly.................................2
    Soma narrow; esophagus without median bulb; but head not as above, female gonad
    always  monodelphic...................................................................................................3

2- Head with separated lips, their edges strongly cuticularized; cheilorhabdion not
    cuticularized (Fig. 1-4) ........................................ TERATOCEPHALOIDEA ........40
    Head and lips not as above but with 6 simple or cirrated bristles (Fig. 5-11);
    cheilorhabdion prominent ..................................... CHAMBERSIELLOIDEA.........43

3- Esophagus cylindrical, not divided into different parts... DRILONEMATOIDEA. ......7
     Esophagus divided into three parts: corpus, isthmus and basal bulb .............................. 4

4- Stoma narrow, structural parts equally cuticularized; gonad, posterior to vulva,
     reflexed twice........................................................CEPHALOBOIDEA....................8
             Two families:
       - Vulva near anus ............................................................ family Metacrobelidae
            Only one subfamily ...............................................................Metacrobelinae
            Only one genus  (Fig. 1) .............................................................Metacrobeles
      - Vulva far from anus................................................family Cephalobidae...........8
   Stoma not evenly cuticularized; gonad posterior to vulva not reflexed twice ...................5

5- Mouth ventrally directed and bordered laterally by membranous
    fins, dorsally by truncate appendage ............................ELAPHONEMATOIDEA
         One family ..........................................................................Elaphonamatidae
         One subfamily.......................................................................Elaphonematinae
         One genus  (Fig.2).......................................................................Elaphonema
    Mouth not as above ..................................................................................................6

6- Bursa present with very large ribs; spicules absent (Fig. 3)......MYOLAIMOIDEA
         One family ...............................................................................Myolaimidae
         One subfamily ..........................................................................Myolaiminae
         One genus  (Fig.3).........................................................................Myolaimus
     Bursa absent, sexual papillae small; spicules present.. PANAGROLAIMOIDEA....28

DRILONEMATOIDEA

FamilyOsstellidae or Drilonematidae

7- Stoma with rhabdions fused to form a tube projected beyond
    labial level; lips transformed into 4 hooks (Fig. 4 ).....................................Osstellinae
         One genus  (Fig.4).............................................................................Osstella
    Stoma absent; head region rounded, smooth; lip normal  (Fig. 5).......Drilocephalobinae
        One genus  (Fig.5)..................................................................Drilocephalobus

CEPHALOBOIDEA

Family Cephalobidae

8- Head with probolae.............................................................................................9
    Head without probolae.......................................................................................10

9- Cephalic probolae large, longer than labial probolae
     (Fig. 7)............................................................................Kirjanoviinae..................11
     Cephalic probolae small, always shorter than labial probolae (Fig.10 )..Acrobelinae..12

10- Cheilostom and prostom wider and rhabdions more heavily cuticularized than following
      parts of stoma (Fig.6 )...........................................................Panagrocephalinae
      Only one genus (Fig.6)............................................................Panagrocephalus
      Only cheilostom wider than other parts of stoma (Fig. ).............Cephalobinae.........25

11- (9) Six paddle-like (or ear-like) cephalic probolae; post vulval sac absent
        (Fig.7).........................................................................................Kirjanovia
            Three paddle-like cephalic probolae; post vulval present
        (Fig.8)................................................................................Acromoldavicus

Subfamily Acrobelinae

12)- Labial probolae furcate (Fig.10 )......................................................................13
      Labial probolae not furcate (Fig.17 )..................................................................20

13)- Labial probolae well cirrated (Fig.10 )..............................................................14
        Labial probolae not cirrated (Fig.11 )...............................................................15

14 (13)- Pointed flap-like cephalic probolae absent; basal portion of labial probolae wollen,
              projecting toward cephalic probolae  (Fig.9)..............................Nothacrobeles

              Pointed flap-like cephalic probolae present; basal portion of labial probolae not
              swollen  (Fig.10)......................................................................Acrobeles

15 (13)- Annules and longitudinal ridges forming blocks all over the body
             (Fig. )...............................................................................................16
               Body without longitudinal ridges, or at most, only in neck region
               ......................................................................................................17

16 (15)- Labial probolae with T- or Y-shaped terminus; female tail
             conical  (Fig.11)....................................................................Stegelletina
              Labial probolae with arcuate, fork-like terminus; female tail broadly rounded
             (Fig.12)................................................................................Stegelleta

Stegelleta labial region

18 (17)- Cephalic probolae with serrate cuticularized borders  (Fig.14) ...........Cervidellus
              Cephalic probolae without serrate borders..................................................19

19 (18)- Labial probolae very thin, hair-like, Y- or T-shaped; head edges cuticularized as in
              Teratocephalus  (Fig.15).......................................................Ypsilonellus
              Labial probolae fork-like; head edges not as above  (Fig.16)........Acrobelophis

20 (12)- Body with plate-like segments (at least in anterior part) (Fig. )........................21
              Body without plate-like segments...................................................................22

21 (20)- Cephalic probolae long pointed; labial probolae low  (Fig.17)..............Scottnema
              Cephalic probolae short, semicircular, lobe-like  (Fig.18)...................Placodira

22 (20)- Cephalic probolae flap-like with dentate projection at
              base; notches between cephalic probolae deep; post vulvar sac absent
           (Fig.19)...................................................................................................Zeldia

23 (22)- Female tail usually long, pointed; cephalic probolae 3, setae-like; labial probolae low,
              rounded  (Fig.20)....... ......................................................Pseudacrobeles
             Tail short, bluntly rounded; cephalic probolae always lower than labial
             probolae....................................................................................................24

24 (23)- Labial probolae, at least the dorsal one, furcate; esophageal corpus almost cylindrical
          (Fig.21)..............................................................................Chiloplacus

              Labial probolae low, rounded or conical, but never furcate; esophageal corpus with
              elongate, spindle-like swelling  (Fig.22).....................................Acrobeloides

25 (10)- Head offset, notched; labial edges cuticularized (as in Teratocephalus)
        (Fig.23)................................................................................Teratolobus
            Head not offset and labial edges not as above.............................................26

26 (25)- Six lips sharply pointed and equal in size  (Fig.24).......................Eucephalobus
              Three lips, never sharply pointed, asymmetrical........................................27

27 (26)- Tail bluntly rounded, sometimes with mucron  (Fig.25)....................Cephalobus
              Tail slender, conoid, pointed  (Fig.26) ...............................Heterocephalobus

PANAGROLAIMOIDEA

Keys to Families, Subfamilies and Genera:

28 (6)- Stoma tubular, its cuticularized portion at least twice as long as wide
            (Fig. 27)...............................................................................................29
             Stoma not tubular, its cuticularized portion about as long as wide
             (Fig. 28, 29)......................................................Panagrolaimidae..... 30

29 (28)- Stoma very long and narrow; vulva at mid body; postvulvar sac
              present.......................................................................... Alirhabditidae
                    Only one subfamily.....................................................Alirhabditinae
                    Only one genus  (Fig.27)..................................................Alirhabditis
              Stoma not very long, relatively wide; vulva near anus; postvulvar sac
              absent.............................................................................Brevibuccidae
                    Only one subfamily ...........................................Brevibuccinae......38

Family Panagrolaimidae:

30 (28)- Esophagus with bulb-like median swelling (Fig.29 )....Tricephalobinae....33
             Esophagus without bulb-like median swelling.............................................31

31 (30)- Promesostom short, metarhabdion with several denticles; anterior end of spicula
              usually hooked  ventrally..............................................Panagrellinae......34
              Promesostom well developed; anterior end of spicula not hooked..............32

32 (31)- Spicula S-shaped, slender (Fig. 28 ) ........................................Turbatricinae
              Only one genus  (Fig.28)........................................ .................Turbatrix
              Spicula not S-shaped, stouter ................................Panagrolaiminae .....35

33 (30)- Distal end of ovary reflexed; postvulvar sac absent; very
              small nematodes (0.25-0.50 mm)  (Fig.29)...........................Halicephalobus
              Distal end of ovary usually passing anus, not reflexed; postvulvar sac present; body
              longer than 0.50 mm  (Fig.30)............................................Tricephalobus

34 (31)- Spicules with bifid tips, proximally knobbed often hooked
              ventrally; postvulvar sac present  (Fig.31) ................................Panagrellus
              Spicules with non-bifid tips, proximal end not hooked; postvulvar sac absent
          (Fig.32).......................................................................Anguilluloides

35 (32)- Six lips divided by deep grooves, their edges cuticularized.........................36
              Labial region without deep grooves, their edges not cuticularized................37

36 (35)- Tail long, 7-8 times as long as anal body widths; postvulvar sac absent
          (Fig.33).....................................................................Panagrobelium
              Tail shorter, 3 times as long as anal body widths; postvulvar sac present
          (Fig.34).......................................................................Panagrobelus

37 (35)- Posterior end of corpus swollen as a median bulb; pro- and mesorhabdions separated
          (Fig.35).....................................................................Procephalobus
              Posterior end of corpus not swollen; pro- and mesostom fused to form a chamber
          (Fig.36)....................................................................Panagrolaimus

Family Previbuccidae:

38 (29)- Body with sheath; inner cuticle with longitudinal rows of pores; six lips separated by
              deep grooves  (Fig.37).......................................................Cuticonema
              Not as above..............................................................................................39

39 (38)- Spicules unequal; female tail long, hair-like (Fig.38)...................Brevibucca
              Spicules equal; female tail shorter, not hair-like (conoid) ( Fig.39)
              ...................................................................................Plectonchus

TERATOCEPHALOIDEA

One family: Teratocephalidae

40 (1)- Gonad single; amphids small, at beginning of stoma: Teratocephalinae........41
            Gonads paired; amphids large, behind stoma: Metateratocephalinae...........42

41 (40)- Cuticle with punctation; vulva at about 40% of body
          (Fig.40).................................................................Steratocephalus
              Cuticle without punctation; vulva at about 65% of body
          (Fig.41)................................................................ Teratocephalus

42 (40)- Head not offset with four fine setae; esophagus, anterior to esophageal bulb,
              with isthmus  (Fig.42)............................................ Euteratocephalus
              Head well offset without setae; esophagus, anterior to esophageal bulb,
              cylindrical  (Fig.43) ............................................ Metateratocephalus

CHAMBERSIELLOIDEA

One family: Chambersiellidae

43 (1)- Cephalic setae with cirri (Fig. 44-46): ..............Chambersiellinae.........44
            Cephalic setae without cirri: ........................Macrolaiminae.................46

44 (43)- Head with two lateral setae (Fig.44)  ...................................Bicirronema
              Head with 6 setae ...............................................................................45

45 (44)- Gonad single  (Fig.45)...................................................Chambersiella
              Gonads paired  (Fig.46). ..................................................Geraldius

46 (43)- Gonad single ......................................................................................47
              Gonads paired ....................................................................................48

47 (46)- Tail long, filiform  (Fig.47)...........................................Macrolaimellus
              Tail short, conical  (Fig.48)...........................................Macrolaimus

48 (46)- Head with 6 cephalic setae and 6 smaller subcephalic setae
          (Fig.49).. ..................................................................Cornilaimus
              Head with 6 cephalic setae only  (Fig.50)..........................Diastolaimus