Bilateral symmetry : Symmetry where only one plane, the median plane (Henle, 1855), divides the body into roughly mirror image right and left halves, at least when viewed from the exterior. Aristotle in De Partibus Animalium laid the foundation for describing animal symmetry and positional information; see translation by Ogle (1912, e.g., 667b), "the body of every sanguineous animal that is capable of locomotion is bilateral; for in such animals there is a distinguishable before and behind, a right and left, an above and below". In Historia Animalium he clarified the problems with positional information in comparative anatomy, "In man, above all other animals, the terms ‘upper' and ‘lower' are used in harmony with their natural positions; for in him, upper and lower have the same meaning as when they are applied to the universe as a whole. In like manner the terms, ‘in front', ‘behind', ‘right' and ‘left', are used in accordance with their natural sense. But in regard to other animals, in some cases these distinctions do not exist, and in others they do, but in a vague way. For instance, the head with all animals is up and above in respect to their bodies; but man alone, as has been said, has, in maturity, this part uppermost in respect to the material universe"; see translation by Thompson (1910, 294a-25). Also see Willmer (1990, p. 15), Brusca & Brusca (1990, p. 46).

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