Indeed, I think there are vestiges of this in our language today. A starfish is not a fish, at least not in the 'proper' sense, but since they're star shaped a

Indeed, I think there are vestiges of this in our language today. A starfish is not a fish, at least not in the 'proper' sense, but since they're star shaped animals that live in the water they are logically called 'starfish.' Jellyfish, cuttlefish, and shellfish are more examples. You may find it interesting that Melville/Ishmael uses walruses as an example of an animal that isn't a fish, because they are amphibious.