I am not a leech, I’m a free living flatworm, meaning I don’t need you (a host) to survive. I don’t get that big, maybe a cm or more. I was just pulled out of the Mississippi near St. Paul, and slapped on this microscope for you to analyze. My fake eyes are special, I can’t see you exactly, but I know you’re there based on your shadow and light. My eyes are pigment spot ocelli which contain photoreceptor proteins that sense light. Eye spots are even found in unicellular organisms, to help in the detection of light for the purpose of photosynthesis. Many scientists, including Darwin, believe that eye spots were a step in the evolutionary chain of what the human eye is today.
My eyes not only have a white pigment spot that senses light, but also a black retina spot that can kinda tell the intensity and the direction of light. Pretty good for fake eyes, huh? Here’s me avoiding the light.