Simple animals? Okay, first, there are no simple animals. They are extreme in their own way, you just cant see them. Like dogs, they have better hearing then us humans and can see in the dark. And Bees, they dont see the world like us they see it in ultra-violt.

And cats. They can squeez into a hole the size of your palm.

So dont ask what is the simpliest animal, because we humans are kind of below the talents animals have.

All animals have a nucleus in each cell. Animals also are able to move during at least one stage in their life. All animals are consumers. Lastly, most animals are multicellular, except for some types of Myxozoa.

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and usually multicellular(although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls. All animals are motile,if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals.

Unlike the largest animal, the "smallest animal" is a title which has not been fully defined, as it is both difficult to identify the size of a microscopic organism compared to another organism. I would say though, Myxozoa is probably one of the closest answers you can get. These microscopic Cnidarians are parasites of fish and were once thought to be Protozoa. The smallest known species are under a hundredth of a millimeter. Another possible candidate would be the Rotifer, while they are 5 times larger than Myxozoans , they are non-parasitic and often free-swimming.