Gas exchange
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Alien Empire tracheal system
Short Youtube clip showing the inside of the tracheal system.
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Fish breathing
Short introductory Youtube clip on fish breathing.
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Fish ventilation
Animations showing countercurrent mechanism and pressure changes in fish.
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Fish ventilation
Straighforward animation to explain countercurrent mechanism from K-science site.
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Gas exchange in animals
Includes worms, fish and insects.
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Gas exchange in fish
From S-cool site, page of information with animations on gas exchange in fish, includes countercurrent mechanism.