A coelom is a fluid-filled body cavity found in many animals, including those in the group known as coelomates. This cavity is located between the digestive tract and the outer body wall, providing space for the development and movement of internal organs. The coelom is lined by a tissue called mesoderm, which also gives rise to various structures in the body. The presence of a coelom allows for greater complexity in organ systems and can facilitate more efficient movement and growth. Animals with a coelom include mollusks, arthropods, and chordates, distinguishing them from simpler organisms like flatworms, which lack a true coelom.