'rigid-shelled' originates from the combination of 'rigid' and 'shell,' where 'rigid' means 'stiff or unyielding' and 'shell' refers to a hard outer covering.

'Rigid' comes from the Latin word 'rigidus,' meaning 'stiff,' and 'shell' comes from the Old English 'scell,' meaning 'a hard outer covering.'

Initially, 'rigid' meant 'stiff or unyielding,' and 'shell' referred to any hard outer covering. The combination 'rigid-shelled' specifically describes organisms with hard outer shells.