Sponges are some of the simplest animals on earth. why do they fit the definition of animals? why are sponges different from plants or fungi?

 The sponge is one of the world's simplest multicellular living organisms. Sponges fit in the definition of animals because they are multicellular and are more closely related to animals with many cell types (Eumetazoa, and Bilateria) than to any protists. Sponges have very interchangeable cells, which means that if you get a bunch of sponge cells and whirl them around in a blender, then put them back in a small container, they will re-form.Sponges differ from plants or fungi because they don't use photosynthesis to create their own food (thus they're not plants) and they filter feed (they are not fungi).