Anopla

A group of marine worms (nemerteans) that lack a proboscis sheath or armor-like structures, representing the most primitive lineage of ribbon worms.

From Ancient Greek 'an-' (without) + 'hopla' (arms, weapons, armor). The term was coined in the 19th century to classify nemertean worms based on their lack of specialized defensive structures compared to other ribbon worm groups.