Congratulations! You seem to already be an expert in homologies — you've
selected all the homologous limbs. The four limbs you picked out all belong
to tetrapods — animals with four legs.
Notice how these tetrapod limbs are similar to one another:
They are all built from many individual bones.
They are all spin-offs of the same basic bone layout: one long bone attached
to two other long bones.
Here you can see the same bones labeled in these different limbs:
Even though these limbs are similar to one another, the animals they belong to
are quite different from one another. What animals possess these homologous limbs?
