Final answer:
Planarians have eyespots that sense changes in light and chemical gradients rather than true eyes. These photosensitive cells are connected to a ladder-like nervous system, aiding in environmental navigation and survival behaviors.
Step-by-step explanation:
Planarians do not have true eyes; instead, they have eyespots that sense changes in light and chemical gradients. These aren't eyes as we think of them; rather, they can only detect light and darkness and are sensitive to changes in the intensity and direction of light. A planarian's eyespots are part of a broader sensory nervous system, which includes nerve cells and sensory organs in their distinct head region, a trait known as cephalization. This feature, along with their bilateral symmetry, evolved to help these organisms navigate and respond to their environment.
These delicate sensory structures are connected to the planarian's ladder-like nervous system, which includes two interconnected parallel nerve cords running the length of the body and cerebral ganglia at the anterior end. This arrangement allows the planarian to process information from its environment, such as the presence of light, which is crucial for survival in their aquatic habitats.
While planarians lack complex visual organs like image-forming eyes that have a cornea, lens, and retina, their photosensitive cells still help them avoid predators and seek out areas that are optimal for feeding and reproduction. This simple yet effective sensory modality showcases the wide range of adaptations organisms have developed to interact with their world.