The document outlines the key internal organs of a fish and their functions, including the brain which processes sensory information, the spine which provides structure, and the heart which circulates blood. The spinal cord relays messages between the brain and body while the stomach, intestines, and pyloric caeca break down and absorb nutrients from food. The liver, kidneys, gallbladder, and gonads also play important digestive and hormonal roles to support the fish's bodily functions.