Digestive system

• Single-celled organisms and sponges digest their food intracellularly and lack a digestive tract
• Other animals digest their food extracellularly within a digestive cavity where digestive enzymes are continuously released
• In coelenterates like Hydra, the gastrovascular cavity serves as both mouth and anus and has only one opening
• Specialization in digestion occurs when the digestive tract has a separate mouth and anus for one-way transport of food
• Nematodes (roundworms) have the most primitive digestive tract, which is a tubular gut lined by an epithelial membrane
• Earthworms have a specialized digestive tract with different regions for ingestion, storage, digestion, and absorption of food
• All higher animal groups, including vertebrates, show similar specializations in their digestive tracts