In zoology, the gut, also known as the alimentary canal or gastrointestinal tract or intestine, is a tube by which bilaterian animals (including humans) transfer food to the digestion organs. In large bilaterians, the gut 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 guts are classified as either protostomes or deuterostomes, as the gut evolved twice, an example of convergent evolution. They are distinguished based on their embryonic development. Protostomes develop their mouths first, while deuterostomes develop their mouths second. Protostome include arthropods, molluscs, and annelids, while deuterostomes include echinoderms and chordates. The gut contains thousands of different bacteria, but humans can be divided into three main groups based on those most prominent. For more specific information on digestive organs, see specialized organs and behaviours.