Why do octopuses have 3 hearts?

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Tags

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