Well reasoned, and not so far off.rjlittlefield wrote:There has to be some structure to the eyespot, or it would not provide direction. One possibility is a lump of units that are individually non-directional, but dense enough to shadow units on the side away from the light. I wonder if that's how it actually works?
--Rik
In fact, the eyespot itself is not photosensitive. It is a sort of parasol that shades the actual photosensitive organ, the very inconspicuous "paraflagellar swelling" (or "paraxial body"), which sits at the base of the emergent flagellum, down inside the gullet of the euglenid. Orientation toward the light is accomplished by parallax. When the photoreceptor is directly exposed to the light, the euglenid turns until the pigmented spot blocks the sun, and it starts swimming or crawling in that direction.Fabulous image! It might be Euglena mutabilis (a very changeable species, as the name suggests). It is certainly not the relatively stiff E. acus (which is a Lepocinclis, in any case).