a) A is the pupil. Its function is to regulate the amount of light entering the eye*.

Here are the answers to Question 15: 15.1 a) A is the pupil. Its function is to regulate the amount of light entering the eye*. b) C is the retina. Its function is to detect light and convert it into electrical signals* that are sent to the brain. 15.2 This question seems incomplete, but the following parts (15.3 a and b) provide the context for explaining differences in nocturnal animals. 15.3 a) B is the lens. In nocturnal animals, the lens is often larger to gather more light in dim conditions, and it may have a greater curvature* to focus more light onto the retina. b) C is the retina. In nocturnal animals, the retina has a higher concentration of rod cells (for low-light vision) and a lower concentration of cone cells (for color vision). Many nocturnal animals also possess a tapetum lucidum* behind the retina, which reflects light back through the photoreceptors, enhancing light detection. 15.4 When a person is exposed to bright light, the iris controls the amount of light entering the eye through a reflex called pupillary constriction. The circular muscles within the iris contract, while the radial muscles relax. This action causes the pupil (part A) to become smaller, thereby reducing the amount of light that can pass through to the retina, protecting it from overstimulation and damage. That's 2 down. 3 left today — send the next one.