Sponges (phylum porifera) are technically members of the animal kingdom. How exactly this is the case is, to the say the least, unclear. They lack tissues, muscles, nerves, and organs, and they feed by sitting there and waiting for nutrients to drift through their bodies.
Sure sounds like a plant, doesn’t it?
Are our expectations now so low that we’ll let anything into the animal kingdom with no questions asked?
This is nothing against sponges per se; the main problem with them is that there is no there there.