A result from my fishing from yesterday: This is a so called "Müller's larva" from a member of the marine Platyhelmintes ("flatworms") (cfr. http://188.8.131.52/detail.php?sp=PLATYHELMINTHES.
Picture Nr. 1: general aspect (ventral side); obj. 10x, DIC; lenght of the larva 500 µm
picture Nr. 2: the pharyngeal opening of the animal; Ob.20x, DIC.
picture Nr. 3: on the surface are green spots, maybe these are symbiotic algae, but I am not sure about it; Obj. 40x DIC.
Images made through a microscope. All subject types.