Final answer:
The spiral valve is found in the small intestine and serves to increase surface area for nutrient absorption. It is present in certain fish species like sharks and rays. Food passing through the spiral valve has enhanced contact with the intestinal wall, aiding in absorption.
Step-by-step explanation:
The spiral valve is found in the small intestine, specifically in the ileum. It is a structure that resembles a corkscrew and is designed to increase the surface area for absorption of nutrients. The spiral valve is found in certain fish, such as sharks, rays, and chimeras. As food passes through the spiral valve, it undergoes additional contact with the intestinal wall, allowing for more efficient absorption.