1. Structural
   1. flippers of a penguin
   2. hooves of a horse
   3. spines of a cactus
   4. hollow bones of a bird
   5. wings of a sea gull
   6. sticky tongue of a frog
2. Functional
   1. the blood chemical, hemoglobin, that carries oxygen
   2. saliva digesting carbohydrates
   3. the poison venom of a snake
   4. chemical of an egg cell that attracts a sperm cell
   5. webbed feet of a duck
   6. protein web made by a spider
3. Behavioral
   1. hibernation of a bear
   2. robber fly mimicking bumble bee not to be eaten by a predator
   3. migration of birds