Animals are classified using several fundamental structural criteria: levels of organization (cellular, tissue, organ, organ-system), body symmetry (radial, e.g., starfish; or bilateral, e.g., most animals including humans), and coelom (a fluid-filled body cavity lined by mesoderm), classifying animals as acoelomate, pseudocoelomate, or coelomate.

• ›Bilateral symmetry is associated with more complex, directional movement and the development of distinct head/tail and back/belly orientation
• ›The coelom's presence and type is a major branch point in animal classification, since a true coelom provides space for organ development and movement