English: Two hypotheses of the different characters and organ systems of Urbilateria: “Complex” Urbilateria, and “Planuloïd” Urbilateria. It is important to note that none of these representations shows an animal which existed or exists, and different combinations of these two organisms can be proposed by some authors (for example, unsegmented Urbilateria with a centralized nervous system). These two representations are only two “extremes” of different hypotheses.