A gill or respiratory organ that lacks a counterpart on the opposite side of the body, found in certain mollusks.
From Greek 'a-' (without) + 'zygon' (yoke, pair) + 'branchia' (gill). The term describes unpaired gill structures in marine organisms.
Many ocean creatures have wildly asymmetrical bodies—some snails have only one working gill, which might seem wasteful but actually helped them evolve into shells with unusual spiral patterns that scientists can trace back millions of years.
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