One key adaptation of animals was the evolution of a(n) , a fluid-filled gap between the outer wall of the body and the outer wall of the digestive system.

The fluid-filled gap is called a body cavity. There are multiple body cavities in the human body, of which include the Cranial cavity, Dorsal cavity, spinal cavity, pelvic cavity, abdominopelvic cavity, abdominal cavity, ventral cavity, and thoracic cavity. The digestive organs are protected by the abdominopelvic cavity.