This is a copy of the author's draft. The definitive version is available at www.blackwell-synergy.comThe structure and optics of the mesopelagic double-eyed mysid crustacean Euchaetomera typica Sars, 1884 are described for the first time.The lateral eye is a typical refracting superposition eye with a wide field of view (172°) and low resolution (interommatidial angle of 7.3°).The antero-dorsal part of the eye is elongated due to the extension of the clear zone.This dorsal eye has a restricted field of view (33°) but much higher resolution (1.5°).The dorsal eye also uses refracting superposition optics, although the optical array is unusual as many of the peripheral ommatidia lack crystalline cones.The centre of curvature of the cornea is in front of the flattened rhabdom layer whereas the axes of the crystalline cones are centred on a point about twice as deep as the rhabdom layer. This results in a well-focused eye, free of spherical aberration. There is a remarkable similarity in eye structure between this species and some mesopelagic double-eyed euphausiid crustaceans
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