An animal has 20 body segments and has no mandibles. which type of animal is it?

It is an annelid.

Annelids are segmented worms. And they do not have mandibles.

The animals that are segmented are vertebrates, arthropods and annelids.

Sure you know that vertebrates have mandibles. But what about arthropods. Well, arthopods include insects, arachnids, myriapods and crustacens. You probably also know that insects and arachnids have mandibles. Myriapods (like the centipedes) and crustaceans also have mandibles. So the answer is annelide.

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