What is the basic body plan of a segmented worm?

The basic body shape of a worm is literally just a tube (the inner makings of the worm) inside of a tube (the worms actual body)

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Explanation:

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these muscles maintain the blood pressure in the vessels. the smooth muscles are innervated by nerves of the autonomic nervous system. when the muscles contract, they cause vasoconstriction of the vessels and increase blood pressure. when the muscles relax, they enable vasodilation of the   blood vessels hence reduce blood pressure.