above the oesophagus receive sensory information from the eyes and statocysts, and further ganglia below control the muscles of the mouth, foot, mantle and viscera. Giant axons up to 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. 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 that has a depth of focus from 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. 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. 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. Squid appear to have limited hearing, 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. Reproductive system The sexes are separate in squid, there being a single gonad in the posterior part of the body with fertilisation being external, and usually taking place in the mantle cavity of the female. The male has a testis from which sperm pass into a single gonoduct where they are rolled together into a long bundle, or spermatophore. The gonoduct is elongated into a "penis" that extends into the mantle cavity and through which spermatophores are ejected. In shallow water species, the penis is short, and the spermatophore is removed from the mantle cavity by a tentacle of the male, which is specially adapted for the purpose