Categorize the structures as homologous or analogous. The mammalian tail and the human coccyx (tail bone) differ in their functions, but the close resemblance of these structures indicates a common evolutionary origin. The flippers of both penguins and dolphins help them swim, but the internal structures of the flippers vary. The function of shells in both turtles and crabs is protection, but the anatomy of their shells is different. The leaves of pitcher plants and the spines of cacti are derived from a common ancestral form, but these structures differ in their functions. The bills of ducks and platypuses have similar functions but differing structures.