'annelidian' originates from New Latin 'Annelida', ultimately from Latin 'annellus' (diminutive of 'annulus'), where 'annulus' meant 'ring'.

'Annelida' (New Latin) was formed from Medieval/Latin 'annellus' and 'annulus' ('ring'); English developed the noun 'annelid' from these forms and the adjective 'annelidian' was derived from that noun.

Initially it referred to being 'ringed' or 'having rings' (from Latin 'annulus'); over time the term came to denote specifically the segmented, ringed worms now called 'annelidians'.