The functional unit of the eye is the photoreceptor cell, which contains the opsin proteins and responds to light by initiating a nerve impulse. The light sensitive opsins are borne on a hairy layer, to maximise the surface area. The nature of these "hairs" differs, with two basic forms underlying photoreceptor structure: microvilli and cilia. In the eyes of protostomes, they are microvilli: extensions or protrusions of the cellular membrane. But in the eyes of deuterostomes, they are derived from cilia, which are separate structures. However, outside the eyes an organism may use the other type of photoreceptor cells, for instance the clamworm "Platynereis dumerilii" uses microvilliar cells in the eyes but has additionally deep brain ciliary photoreceptor cells. The actual derivation may be more complicated, as some microvilli contain traces of cilia – but other observations appear to support a fundamental difference between protostomes and deuterostomes. These considerations centre on the response of the cells to light – some use sodium to cause the electric signal that will form a nerve impulse, and others use potassium; further, protostomes on the whole construct a signal by allowing "more" sodium to pass through their cell walls, whereas deuterostomes allow less through.