Some animals, like jellyfish and simple animals, can function without hearts, brains and other vital organs.
Key Facts In This Video
A major distinction of simple animals is their tissue complexity. 00:50
The first germ layers appeared about 535 million years ago. 05:42
Mollusks are simple animals, yet can be very smart. 08:36
Written by Curiosity Staff February 5, 2015
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