A professor of Evolutionary Biology who goes by the handle DarwinZDF42 argues that something like this kind of circulatory system in one kind of fish:
can evolve to another kind of circulatory system in another kind of fish (a lungfish):
I pointed out he was kind of sketchy on the details of the process of evolving the single loop system of most fish to something like the double loop system of lungfish. Not the least his problems was accounting for a lung that is connected to a heart. If he cites the lungfish system as a transitional to other creatures, he may as well invoke special creation since he’s citing organs like the lung connected to a heart that pop into existence out of nowhere. A real transitional or set of transitionals would be smooth evolutionary between the single loop system and the double loop system.