Digestive system

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