Okay, this dot point is easy
Photoreceptors in human are a thin cell that contain light sensitive pigments. The pigments will distort in shape when the pigment is stimulated by light and initiate an electrochemical signals that can be interpreted by the brain. Remember brain do not repsond to mechanical stimuli, only to electrochemical signal.
Humans have rods and cones. Rods contain rhodopsin and is used to detect movement, outline of the shape and black & white vision in dim light. Cones are used for colour vision and fine detail but operate only in bright light.
Insects have a compound eye. a compound eye contain thoundsands of light detecting unit called ommatilda. each immatidium has its own lens which focues light onto a light absorbing pigment. these pigments are arranged in stacks of plates called rhabdom. the altered pigment initate a nerve impulse, which is transmitted to nerve fibre continous with the receptor cells.
Each ommatidium egisters vision froma different part of the enviroment, the result is an image that is a patter of dots. the visual pigment return to its normal shape so quicly, therefore any chnages to the light pattersn such as movement cna be registered very quickly.
The planarian worm, the structure is called an eye cup. which holds cells containing photreceptors. when these photoreceptors are stimulated by light, they alter so that a nerve impulses is sent to the brain. there re two cups, thus when the brain registers light intensity and direction, the animal move around and away from the ligh source toward an area of low light intensity. The darkned helps to protect the planarian worm from predators.
Hope this help
