The “shell” is « actually a craddle […] secreted by the two dorsal arms of the female. It is used for the purpose of protecting the tiny eggs. » (Archerd Shell Collection/Shell Classes/Cephalopoda/Argonautidae).
Argonauta Linnaeus, 1758:
« Shell free, univalve, carrycot-shaped; flattened keel on the periphery, tuberculate on both sides; mouth entire, receiving in its middle the return of the whorl. » – P. D. de Montfort: Conchyliologie systématique vol.II, Paris 1810, p.7.
