an electric source, perhaps by comparing differences in signal strength across the sheet of electroreceptors. This would explain the characteristic side-to-side motion of the animal's head while hunting. The cortical convergence of electrosensory and tactile inputs suggests a mechanism that determines the distance of prey that, when they move, emit both electrical signals and mechanical pressure pulses. The platypus uses the difference between arrival times of the two signals to sense distance. Feeding by neither sight nor smell, the platypus closes its eyes, ears, and nose each time it dives. Rather, when it digs in the bottom of streams with its bill, its electroreceptors detect tiny electric currents generated by muscular contractions of its prey, so enabling it to distinguish between animate and inanimate objects, which continuously stimulate its mechanoreceptors. Experiments have shown the platypus will even react to an "artificial shrimp" if a small electric current is passed through it. Monotreme electrolocation probably evolved in order to allow the animals to forage in murky waters, and may be tied to their tooth loss. The extinct Obdurodon was electroreceptive, but unlike the modern platypus it foraged pelagically (near the ocean surface). Eyes In recent studies it has been suggested that the eyes of the platypus are more similar to those of Pacific hagfish or Northern Hemisphere lampreys than to those of most tetrapods. The eyes also contain double cones, which most mammals do not have. Although the platypus's eyes are small and not used under water, several features indicate that vision played an important role in its ancestors. The corneal surface and the adjacent surface of the lens is flat while the posterior surface of the lens is steeply curved, similar to the eyes of other aquatic mammals such as otters and sea-lions. A temporal (ear side) concentration of retinal ganglion cells, important for binocular vision, indicates a role in predation, while the accompanying visual acuity is insufficient for such activities. Furthermore, this limited acuity is matched by a low cortical magnification, a small lateral geniculate nucleus and a large optic tectum, suggesting that the visual midbrain plays a more important role than the visual cortex, as in some rodents. These features suggest that the platypus has adapted to an aquatic and nocturnal lifestyle, developing its electrosensory system at the cost of its visual system; an evolutionary process paralleled by the small number of electroreceptors in the short-beaked echidna, which