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Just as algae gained by living together in colonies, so protozoa began living together. The earliest many-celled animals might have been sponges, which are bottle-shaped animals which look like plants. A current of water is drawn in through small holes in their sides by the beating of tiny whip-like hairs (flagella). Food is filtered out of the water current, which leaves through a large hole at the top.
Sponges might have evolved into jellyfish. Sponges might have appeared around 700 million years ago. |
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