An organ for vision must be formed of transparent tissue, and must include some sort of lens for throwing an image at the back of a darkened chamber.

Beyond this superficial resemblance, there is hardly any real similarity between the eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's admirable memoir on these organs in the Cephalopoda.

It is impossible for me here to enter on details, but I may specify a few of the points of difference.

The crystalline lens in the higher cuttle-fish consists of two parts, placed one behind the other like two lenses, both having a very different structure and disposition to what occurs in the vertebrata.

The retina is wholly different, with an actual inversion of the elemental parts, and with a large nervous ganglion included within the membranes of the eye.