Final answer:
Manatees and cetaceans share homologous structures in their forelimbs, which have a similar set of bones that have been adapted to their aquatic lifestyles due to divergent evolution from a common ancestor. This differs from analogous structures, which arise from similar environmental pressures but do not indicate a common evolutionary past.
Step-by-step explanation:
The common feature that manatees and cetaceans (such as whales and dolphins) share regarding their forelimbs is that they are homologous structures. This means that despite having different forms and functions, the forelimbs have a similar underlying structure which points to a common evolutionary ancestor. In manatees, these structures function as paddles for moving through water, while in cetaceans, they have evolved into flippers for swimming. Both manatees and cetaceans have forelimbs with bones corresponding to the humerus, radius, ulna, and sometimes even fingers, although highly modified.
These similarities occur because of divergent evolution, where species sharing a common ancestor evolve and adapt to different environments, causing variation in the structure and proportion of their limbs. The adaptation of their limbs is a response to their aquatic lifestyles, though they evolve independently, reflecting their separate evolutionary paths from a common ancestor. This contrasts with analogous structures, where organisms develop similar features independently, not because they share an ancestor, but because they live in similar environments and face similar selective pressures, such as dolphins and fish both having streamlined bodies for efficient swimming despite having very different evolutionary histories.