KEY TAKEAWAYS •
A circulatory system is necessary when diffusion is insufficient in getting oxygen to all animal tissues.
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The respiratory system is different in different animals but involves structures that exchange oxygen and carbon dioxide.
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The animal digestive system involves ingestion, mechanical digestion, chemical digestion, absorption, and elimination of waste.
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Animal organisms will have an innate and sometimes an adaptive immune system, the latter of which is evolutionarily younger.
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The endocrine system involves endocrine glands that send hormones through the circulatory system to act on distant organs.
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Reproduction can be asexual or sexual in animals, with a variety of mechanisms of reproduction possible.
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