The posterior-most region of an annelid worm that contains the anus and often bears sensory structures or reproductive organs. This terminal section is not considered a true segment and grows throughout the animal's life.
From Greek 'pygidion,' a diminutive of 'pyge' meaning 'rump' or 'buttocks.' The term was adopted into zoological nomenclature in the 19th century to describe the tail region of segmented worms, emphasizing its position at the animal's rear end.
The pygidium is proof that worms need to watch their backs too - it's often loaded with sensors and can even regenerate if lost to a predator! Some marine worms can actually swim backwards using their pygidium to steer, making it both a tail and a rudder.
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