Why does an octopus have 3 hearts?

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Tags

1. #contains
2. #copper
3. #octopuses
4. #transport
5. #systemic
6. #through
7. #circulates
8. #haemocyanin
9. #heart
10. #three
11. #octopus
12. #oxygen
13. #branchial
14. #blood
15. #gills
16. #round
17. #protein
18. #hearts