Barnacles and limpets (shown below) have many superficial similarities: both are small creatures with conical shells and can be found in the ocean on rocky shores.
But the similarities end there. Inside their shells, they are very different:
What do these differences mean? They suggest that barnacles and limpets are not very closely related, and that their common ancestor (probably a worm-like animal) did not have a conical shell. Their similar shells evolved because both lineages faced a similar challenge: a rocky, wave-swept environment teeming with predators ready to pry a small creature off a rock for lunch — and conical, hard-to-get-a-grip-on armor offers a lot of protection in such an environment. These lineages evolved their shells independently — meaning that the shells of these two lineages are analogies.