VISION. 493 



the class Crustacea, are placed at the ends 

 of moveable pedicles, so as to admit of being 

 turned at pleasure towards the objects to be 

 viewed.* This, however, is not the case with 

 the Entomostraca, comprising the various species 

 of Monoculi, in which the two eyes are brought 

 so close to one another as apparently to consti- 

 tute a single organ, corresponding in its struc- 

 ture to the fourth class of eyes already enume- 

 rated ; that is, the separate lenses it contains 

 have a general envelope of a transparent mem- 

 brane, or cornea. Muscles are provided for 

 moving the eye in its socket ; so that we have 

 here indications of an approach to the structure 

 of the eye which prevails in the higher classes 

 of animals. There is, however, a still nearer 

 approximation to the latter in the eye of the 

 Cephalopoda ; for Sepia differ from all the 

 tribes belonging to the inferior orders ofmollusca 

 in having large and efficient eyes, containing a 

 hemispherical vitreous humour, placed imme- 

 diately before a concave retina, and receiving in 

 front a large and highly convex crystalline lens, 

 which is soft at its exterior, but rapidly increases 

 in density, and contains a nucleus of great hard- 

 ness; there is also a pigmentum nigrum, and a 



* Latreille describes a species of Crab, found on the shores of 

 the Mediterranean, having its eyes supported on a long jointed 

 tube, consisting of two articulations, which enables the animal 

 to move them in various directions, like the arms of a telegraph. 



