Analogy: Of Shrimp and Snails
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:
|Barnacles have the body layout of a shrimp.
||Limpets have the body layout of a snail.
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.