Senses are adaptive.
Sensory receptors may form sense organs. A sensory receptor selectively responds to a single form of energy and converts it to receptor potentials, which change membrane potential in proportion to stimulus strength. Sensory receptors include chemoreceptors, photoreceptors, and mechanoreceptors. Invertebrates have chemoreceptors to detect food and perform other functions. Invertebrate photoreceptors contain pigments associated with membranes.