Planarian flatworms are simple organisms recognized for their “cross-eyed” appearance. This look comes from two primitive eyes, known as eyespots or ocelli, on the upper side of their head. Unlike complex vertebrate eyes that form detailed images, a planarian’s ocelli detect the intensity and direction of light. This sensory information guides the worm’s behavior and survival.
Structure and Function of Planarian Eyespots
The planarian eyespot consists of two main cell types. At its core is a crescent-shaped cup made of pigment cells that absorb incoming light. This cup creates a shaded area, preventing light from reaching sensory cells from most directions.
Nestled within this pigment cup are photoreceptor neurons. Since the cup blocks light from the back and sides, only light entering from an unshielded opening can stimulate these neurons. This arrangement allows the planarian to determine a light source’s direction. Axons, or nerve fibers, from these neurons then transmit this information to the brain for processing.
Light-Sensing and Behavior
Information from the eyespots influences the planarian’s movement, a behavior called negative phototaxis, which is the tendency to move away from a light source. To orient itself, the worm swings its head from side to side. This scanning motion allows it to compare the light intensity detected by its left and right eyespots.
By comparing these signals, the planarian determines which direction is darker and moves accordingly. This response is a survival advantage. Moving from light helps the flatworm avoid predators that hunt by sight and guides it toward dark, moist environments to prevent dehydration.
Regeneration of Eyes
A remarkable quality of planarians is their capacity for regeneration, which includes their eyes. If a planarian is decapitated, the head and its eyespots can regrow completely. This ability comes from adult stem cells called neoblasts, distributed throughout the worm’s body. These pluripotent cells can develop into any cell type the worm needs.
Following an injury, neoblasts migrate to the wound site. They then proliferate and differentiate, specializing to become the cells needed to rebuild the missing structures. Eye regeneration involves forming new pigment cup cells and photoreceptor neurons in the correct orientation. The photosensing system begins to re-establish within about four days, with light-avoiding behavior returning shortly after.
Scientists study the genetic pathways controlling this process, such as the expression of the gene opsin, which is involved in light detection. Other genes guide the formation of the optic cup and photoreceptors. By investigating how these animals rebuild their eyes, researchers hope to uncover principles of organ and tissue regeneration.