A pelagic (ocean-swimming) form of certain polychaete worms that differs in structure from the benthic (bottom-dwelling) form of the same species.
From Greek hetero- (different) and Nereis (a sea nymph), describing the transformed 'different Nereid' appearance of these worms when they switch from bottom life to swimming in the ocean column.
Imagine an animal that looks completely different depending on where it lives—when these worms want to swim in open ocean, they literally remodel their bodies with new fins and eyes, like swapping out their entire outfit!
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