Evidence that the roundworms, protostomes, and deuterostomes evolved from an ancestor that was not an ancestor to the other animals is that all three groups have all have a complete digestive tract . Since this evolutionarily derived trait is not found in other animals, it provides evidence that these groups all evolved from an ancestral species (the species you clicked to get to this page) which evolved this trait.
The digestive tract is said to be complete if food is ingested (taken in) at one end of the tract, the mouth, and wastes from digestion are passed out of the tract at the other end, the anus. There are thus two openings (mouth and anus) to a complete digestive tract. This contrasts with an incomplete digestive tract, which apparently evolved before the complete digestive tract evolved. An incomplete digestive tract has just one opening, so that the same opening was used as both to take in food and to get rid of wastes. Complete and incomplete digestive tracts are shown in the following figure:
Complete and Incomplete Digestive Tracts:
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