| I: | Fishes have a 2-chambered heart with an atrium and a ventricle. |
| II: | Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle. |
| III: | Crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles. |
| IV: | The only vertebrates where the heart pumps out deoxygenated blood are fishes. |
| 1. | 1 | 2. | 2 |
| 3. | 3 | 4. | 4 |