Sea stars draw in water and channel it through canals in their bodies, typically ending in their tube feet.
To better understand how sea stars (also known as starfish) take in water, it's crucial to understand their unique water vascular system. This system is vital for locomotion, feeding, respiration, and excretion. Here's a breakdown:
The Water Vascular System: A Detailed Look
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Madreporite: The process starts with the madreporite, a small, sieve-like plate located on the aboral (top) surface of the sea star. This structure acts as an entry point for seawater.
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Stone Canal: Water enters the madreporite and then passes through the stone canal. This canal is often calcified, hence the name "stone."
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Ring Canal: The stone canal leads to the ring canal, which encircles the mouth.
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Radial Canals: From the ring canal, five radial canals extend outwards, one along the length of each arm.
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Lateral Canals: Each radial canal connects to numerous lateral canals.
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Tube Feet (Podia): The lateral canals lead to the tube feet, also known as podia. These are small, hollow, muscular projections that extend from the oral (bottom) surface of the sea star's arms. They are equipped with suction cups.
The Role of Tube Feet
The tube feet are essential for several functions:
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Locomotion: Sea stars move by coordinating the movement of their tube feet. Water is pumped into the tube feet, causing them to extend. The suction cups then attach to the substrate, and the muscles in the tube feet contract, pulling the sea star forward.
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Feeding: Some sea stars use their tube feet to pry open shellfish. They can exert tremendous force over long periods, eventually tiring out the prey.
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Respiration and Excretion: Gas exchange (oxygen uptake and carbon dioxide release) and the excretion of nitrogenous waste can occur directly across the thin walls of the tube feet.
Summary
In summary, sea stars take in water through the madreporite, which then travels through the water vascular system to the tube feet. This system allows them to move, feed, and perform other vital functions.
