(i) Vertebrate brain: The brain of vertebrates has evolved. Brains are enlarged in fish, amphibians, reptiles, avius and mammals. The structure of the cerebral hemispheres and cerebellum is complex.
(ii) Vertebrate heart: The heart of vertebrates has undergone gradual evolution. The heart of fish is two-chambered, the heart of amphibians is three-chambered, the heart of reptiles is partially four-chambered, and the heart of birds and mammals is fully four-chambered. The heart has progressed from simple to complex.
(iii) Homologous organs: Organs which are similar in function but different in structure and origin are called homologous organs. For example, the wing of a bird and the wing of a butterfly are symmetrical organs.
(iv) Homologous Organs: Organs which are structurally similar are called homologous organs. Wings of birds and bats, hands of humans, front legs of horses and flippers of whales are related organs.
(v) Peripheral organs: Organs which are not functional are called peripheral organs. Such as ear muscles, pineal gland, coccyx and body hair.