定义：a type of locomotion in which an animal moves on a secreted layer of mucus, propelled by the beating of cilia. It is characteristic of small, soft-bodied invertebrates.
定义：Eye-spots which may be set into a pit to reduce the angles of light that enters, to allow the organism to deduce the angle of incoming light. Lens-like structures may be present, but stemmata, unlike lens eyes, cannot form an image, either for lack of refractive power or retinal resolution.
