Mollusca - Complete digestive system with mouth, gut and anus. Their main digestive system is found in the visceral mass which contains other organs used in reproduction and excretion. Cnidaria - One-way digestive tract where undigested food and digested food all pass through the same opening. All digestion happens in the gastrovascular cavity. Echinodermata - complete digestive system with mouth, gut and anus. Some often eat with their stomachs outside their body.