Key to the species and forms of Hypocreadium

1 Body oval; width less than 70% of length; ovary more or less pretesticular............................................................... 2

- Body closer to circular or wider than long; ovary inter- or post-testicular................................................................. 3

2 Genital pore anterior to intestinal bifurcation; uterus reaches posteriorly to ovary; sucker-ratio 1:1.22 ....................... ...................................................................................................................................... H. symmetrorchis Ozaki, 1936

- Genital pore posterior to intestinal bifurcation; uterus pre-ovarian; sucker-ratio 1:0.51–0.89 ...................................... ..................................................................................................................... H. biminensis (Sogandares-Bernal, 1959)

3 Anterior notch distinct ................................................................................................................................................ 4

- Anterior notch absent or indistinct ............................................................................................................................... 6

4 Vitelline fields in forebody confluent or nearly so; caecal endings directed distinctly anteriorly ................................. ................................................................................................................................... H. toombo Bray & Justine, 2006

- Vitelline fields distinctly separated in forebody; caecal endings usually directed posteriorly, occasionally transverse or slightly anterior ........................................................................................................................................................ 5

5 Uterus always pre-ovarian; genital pore always at bifurcal level; ovary intertesticular................................................. ....................................................................................................................................... H. cavum Bray & Cribb, 1996

- Uterus may reach para- or post-testicularly; genital pore variable, between pharynx and bifurcal level; ovary inter to post-testicular ............................................................................ H. patellare Yamaguti, 1938 (various forms, see text)

6 Body pyriform.............................................................................................................................................................. 7

- Body oval to circular.................................................................................................................................................... 8

7 Narrow gap between vitelline fields and body margins, fields confluent anteriorly...................................................... ........................................................................................................................ H. lactophrysi (Nahhas & Cable, 1964)

- Wide gap between vitelline fields and body margins, fields separated anteriorly ............................. H. picasso n. sp.

8 Vitelline fields confluent in forebody .......................................................................................................................... 9

- Vitelline fields separated in forebody ......................................................................................................................... 10

9 Body oval, width 84–91% of length; vitelline fields confluent in hindbody; uterus pre-ovarian .................................. .................................................................................................................................. H. galapagoensis (Manter, 1945)

- Body probably circular (edges folded); vitelline fields separated in hindbody; uterus para-ovarian ............................ .......................................................................................................................................... H. spinosum (Manter, 1940)

10 Vitelline fields widely separated in hindbody................................................................................................................. H. lamelliforme (Linton, 1907) (this species is poorly known and the entry in this key is based on Fig. 77 in Linton (1907) from the grey triggerfish, Balistes capriscus Gmelin (as B. carolinensis Gmelin) – the first host listed and considered type-host here; fig. 78 represents Dermadena lactophrysi Manter (see Manter, 1945); Fig. 76 also probably represents a Dermadena sp.)

- Vitelline fields confluent in hindbody ........................................................................................................................ 11

11 Caecal ends oriented anteriorly, uterus preovarian.......................................................... H. indicum (Madhavi, 1972)

- Caecal ends oriented posteriorly, uterus postovarian ................................................................................................. 12

12 Body broadly oval, width, 126–149% of body-length ...................................... H. grandiquamis Bray & Cribb, 1996

- Body more nearly circular, width <114% of body-length .......................................................................................... 13

13 Caecal endings transversely oriented; ovary more or less lobed; eggs 51–56 × 32–44 ................................................. .................................................................................................................................. H. scaphosomum (Manter, 1940)

- Caecal endings posteriorly oriented; ovary entire; eggs>64 × 40 ............................................................................. 14

14 Vitelline fields confluent in wide band in hindbody............................................................ H. balistes (Nagaty, 1942)

- Vitelline fields slightly separated in hindbody .................................. H. myohelicatum Bravo-Hollis & Manter, 1957