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Eyes


Nautilus has a near-spherical large eye with no lens and an extensive retina. This eye is effectively a pin-hole camera. It is possible for minimal focusing of the eye as the aperture is variable in size, but there are no other means of focusing and it opens directly into the sea.

When compared to vertebrates, all cephalopods have a retinal arrangement that is reversed, which means that the light hits the receptors first rather than after passing through a layer of nerves. All other cephalopods apart from Nautilus have a large spherical lens within the eye. The retinas of cephalopods, like most other invertebrates, are based on microvillous cells. In squid the retina is composed of groups of four tall cells with a central space, which is filled with microvilli that are oriented at right angles to the adjacent cells and are interwoven to form the rhabdom. It is the rhabdom that provides the basis for sensitivity to polarized light. Visual pigments are membran-bound and based on the same vitamin A chromatophores as found in fishes. Cephalopods have more than one visual pigment, and these are located in different parts of the retina. However, it is believed that cephalopods do not have colour vision.


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Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2004-5