Good point.

I’d like to suggest the swap of oyster in the place of clam, because oysters are less mobile than clams.

This leads to a funny observation: for some reason I think a worm and an oyster are obviously animals, like if you were a caveman with no notion of genetics or the tree of life and you came across either, I suspect you’d think “this thing is obviously some kind of animal.” But a sponge is not so obvious, I think, to our hypothetical caveman. I could believe a sponge is a weird plant.

I think you need at least one distinguishing feature beyond the minimal to become obviously an animal, for some reason.