(from Medicine Chest)
The first animals to have anything like an intestine evolved around 548 million years ago. These were roundworms, just a few millimeters long, and they had just a simple tube running from their mouth to their anus - no stomach in the middle. But already these roundworm guts had tremendously important help in digesting food - inside each roundworm tube lived thousands of bacteria. These bacteria had evolved so that they could only live inside roundworm guts, and they ate some of the food the roundworm caught, but they also helped the roundworm digest the food and get more energy out of it.
Only a few million years later, segmented worms (annelids) had an intestine separate from the crop and the gizzard. The crop stored food, the gizzard
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