| It is able to withdraw the whole of its body into its shell which is then sealed by an amber-coloured operculum. |
| The halecostomes, of which Amia is an example, have an opercular dilator muscle originating on the mandible which opens the operculum. |
| The spine on the operculum is the best way to identify an angelfish and to distinguish it from similar-looking species, such as butterfly fish. |
| The nape lacks cirri, and the first gill arch attaches to the operculum, the latter characteristic distinguishing Tripterygiidae from Clinidae. |
| Avicularia are small heterozooids in which the zooecium and operculum form a beak-like, snapping structure that deters small predators. |
| The chimaeran hyoid is, it turns out, quite happy supporting the operculum and has no interest in the palatoquadrate. |
