Animals are members of kingdom Animalia. One of the unify features of this kingdom is multicellularity. Slime molds are members of the mostly-single-celled protists and they themselves are some of the most complex single celled organisms. Sponges, being very basal animals, are one of the phyla that retains a high degree of regenerative ability that simplicity facilitates while being very much a multicellular organism with all the organization that comes with.