All the other animal phyla are in the Metazoa subkingdom. Individual cell obtain food via a process similar to Protistans. Cellular differentiation means that different cells serve different purposes. Sponges are too complicated to be placed in the Protista kingdom. Porifera Sponges do not eat their food.
Sponges have no muscles, nerves and gut that the other animals possess.
The phylum Porifera is placed in its own subkingdom Parazoa. Sponges are multicellular and display cellular differentiation.