[Campbell Biology P.1166] According to the text and Figure 50.15, if the screening pigment in... | Practice Question

According to the text and Figure 50.15, if the screening pigment in a planarian's ocelli were completely absent, what would be the most likely consequence for its light avoidance behavior?

  • A: The planarian would become hypersensitive to light from all directions and be unable to discern a light source.
  • B: The planarian would lose its ability to detect light entirely, as the photoreceptors would be unprotected.
  • C: The planarian would be attracted to light sources instead of avoiding them.
  • D: The planarian's vision would become clearer, allowing it to navigate more effectively in bright environments.

Explanation

Figure 50.15 b explains that the screening pigment blocks light striking the back of the ocellus. This allows the ocelli to indicate the direction of a light source by comparing the excitation from different angles. If the pigment were absent, light from all directions would excite the photoreceptors equally, preventing the planarian from accurately discerning the light source's direction and thus disrupting its ability to move away from light.