What structural characteristic does a taxonomist look for in classifying an animal?

Taxonomists and other scientists will look for a variety of structural characteristics to classify an animal. Some of the major characteristics are: presence or absence of a backbone (vertebrate versus invertebrate), presence and type of symmetry (bilateral, radial, etc), type of circulatory system (open or closed), adaptations of the extremities (wings, specialized limbs, etc) and structure of the heart (number and orientation of chambers).