Armhook

In Ancistrotheutis the attachment ring persists, and the hook sac surrounding the tentacle hook is closed except during function contrary to the condition in the armhooks where the hook always projects from the sac.

Source: wiktionary

The bivalve Posidonia permica frequently occurs in large concentrations on slabs of the shale in which there are also concentrations of cephaloped prehensile armhooks representing no doubt distorted natural assemblages from single individuals.

Source: wiktionary

Furthermore the armhooks of the Belemnoidea show a micro-texture and a mode of growth similar to the horny inner rings of the suckers of the Decabrachia.

Source: wiktionary

When the partners meet they form armhook with each other, turn around once, let go, and continue the march till they again meet, when they march up into straight flank formation and halt at the end of the stanza.

Source: wiktionary