Key to species of Cucullanus

1 Males and females over 25 mm long............................................................. C. elongatus

- Males and females less than 21 mm long.................................................................. 2

2 Cuticle with prominent transverse annulations anteriorly............................................ C. annulatus

- Cuticle with only fine transverse striations throughout....................................................... 3

3 Males 5.3–9.2 mm, females 7.5–12.3 long; parasites of flatfishes................................... C. heterochrous

- Males 9.5–14.7 mm, females 12.9–20.8 mm long; parasites mainly of Gadidae and Merlucciidae .............. C. cirratus