Editing Squid (section)

===Nervous system and sense organs===
{{see|Cephalopod intelligence}}

Cephalopods have the most highly developed nervous systems among [[invertebrate]]s. Squids have a complex brain in the form of a nerve ring encircling the [[oesophagus]], enclosed in a cartilaginous [[cranium]]. Paired cerebral [[ganglia]] above the oesophagus receive sensory information from the eyes and [[statocyst]]s, and further ganglia below control the muscles of the mouth, foot, mantle and viscera. [[Squid giant axon|Giant axons]] up to {{cvt|1|mm|in|2}} in diameter convey nerve messages with great rapidity to the circular muscles of the mantle wall, allowing a synchronous, powerful contraction and maximum speed in the jet propulsion system.<ref name=Ruppert/>

The paired eyes, on either side of the head, are housed in capsules fused to the cranium. Their structure is very similar to that of a fish eye, with a globular [[Lens (anatomy)|lens]] that has a [[depth of focus]] from {{convert|3|cm|in|1|abbr=on}} to infinity. The image is focused by changing the position of the lens, as in a camera or [[telescope]], rather than changing the shape of the lens, as in the [[human eye]]. Squid adjust to changes in light intensity by expanding and contracting the slit-shaped [[pupil]].<ref name=Ruppert/> Deep sea squids in the family [[Histioteuthidae]] have eyes of two different types and orientation. The large left eye is tubular in shape and looks upwards, presumably searching for the silhouettes of animals higher in the [[water column]]. The normally-shaped right eye points forwards and downwards to detect prey.<ref>{{cite web | author1 = Young, Richard E. | author1-link = Richard E. Young | author2 = Vecchione, Michael | author2-link = Michael Vecchione | year = 2013 | title = ''Histioteuthidae'' Verrill, 1881. | url = http://tolweb.org/Histioteuthidae/19782/2013.11.03 | publisher = The [[Tree of Life Web Project]] |access-date=9 December 2018}}</ref>

The statocysts are involved in maintaining balance and are analogous to the inner ear of fish. They are housed in cartilaginous capsules on either side of the cranium. They provide the squid with information on its body position in relation to gravity, its orientation, acceleration and rotation, and are able to perceive incoming vibrations. Without the statocysts, the squid cannot maintain equilibrium.<ref name=Ruppert/> Squid appear to have limited hearing,<ref>{{cite news|url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_8095000/8095977.stm|title=The cephalopods can hear you |publisher=BBC|date=15 June 2009|first=Matt|last=Walker|access-date=2 April 2010}}</ref> but the head and arms bear lines of hair-cells that are weakly sensitive to water movements and changes in pressure, and are analogous in function to the [[lateral line]] system of fish.<ref name=Ruppert/>