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The tract (or tube) is in bilaterian animals. It carries food through digestion and excretion. Into the tube come various digestive enzymes. Gut flora help digestion, and the production of vitamins. Muscular movements pass the material down the tube. The gut usually has an exit, the anus, by which the animal disposes of solid wastes. Some small animals have no anus and dispose of solid wastes by other means, for example through the mouth.
Animals that have guts are classified as either protostomes or deuterostomes, as the gut evolved twice, an example of convergent evolution. They are distinguished by the development of their embryos. Protostomes develop their mouths first, while deuterostomes develop their mouths second. Protostome include arthropods, molluscs, and annelids, while deuterostomes include echinoderms and chordates.