that picture is a view down some part of the the intestine. the darker part in the center is the hollow of the intestine and gets darker towards the center because the center is further away from the light source. it is the perspective point. it is crescent shaped somewhat because the intestine is shaped like a cylinder and is also bending. the mass of candida on the right that extends into the dark area also contributes to the crescent-like appearance. you are looking down a tube so to speak.