The gastrointestinal tract, also known as the gut or alimentary canal, is a tube by which bilaterian animals (including humans) transfer food to the digestion organs. In large bilaterians, the gastrointestinal tract generally also has an exit, the anus, by which the animal disposes of solid wastes. Some small bilaterians have no anus and dispose of solid wastes by other means (for example, through the mouth).
Animals that have gastrointestinal tracts are classified as either protostomes or deuterostomes. The digestive tract evolved separately in these two clades, an example of convergent evolution. The clades are distinguished based on their embryonic development: protostomes develop their mouths first, while deuterostomes develop their mouths second. Protostomes include arthropods, molluscs, and annelids, while deuterostomes include echinoderms and chordates.
The gastrointestinal tract contains thousands of different bacteria, but humans can be divided into three main groups based on those most prominent in the human gastrointestinal tract.