Editing Platypus (section)

=== 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 [[lamprey]]s than to those of most tetrapods. The eyes also contain [[Double cone (biology)|double cones]], unlike most mammals.<ref>{{cite journal |last1=Zeiss|first1=Caroline|title=Comparative retinal morphology of the platypus|doi=10.1002/jmor.10959 |year=2011 |first2=Ivan R. |last3=Murphy|first3=Christopher J. |last4=Dubielzig|first4=Richard W. |journal=Journal of Morphology |volume=272 |issue=8 |pages=949–57 |pmid=21567446|last2=Schwab|s2cid=28546474}}</ref>

Although the platypus's eyes are small and not used under water, several features indicate that vision was important for its ancestors. The [[cornea]]l 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 bone|temporal]] (ear side) concentration of [[retinal ganglion cell]]s, important for binocular vision, indicates a vestigial role in [[predation]], though the actual visual acuity is insufficient for such activities. Limited acuity is matched by low [[cortical magnification]], a small [[lateral geniculate nucleus]], and a large [[optic tectum]], suggesting that the [[Midbrain tectum|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. This contrasts with the small number of electroreceptors in the [[short-beaked echidna]], which dwells in dry environments, while the [[long-beaked echidna]], which lives in moist environments, is intermediate between the other two monotremes.<ref name="sensory_platypus" />