The five main groups of the animal kingdom are: 1. Porifera (Sponges) 2. Cnidaria (Jellyfish, coral, and sea anemones) 3. Platyhelminthes (Flatworms) 4. Nematoda (Roundworms) 5. Mollusca (Snails, clams, and squids) These five groups are considered the simplest and most primitive forms of the animal kingdom. They are characterized by simple body plans, a lack of specialized organs, and limited mobility. They form the foundation of the animal kingdom, and their anatomy and physiology have been modified and diversified in the more advanced animal groups that evolved from them.