VISION. 441 



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 ap- 

 parently to constitute a single organ, corresponding 

 in its structure to the fourth class of eyes already 

 enumerated ; that is, the separate lenses it contains 

 have a general envelope of a transparent membrane, 

 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 Sepice differ from 

 all the tribes belonging to the inferior orders of mol- 

 lusca in having large and efficient eyes, containing 

 a hemispherical vitreous humour, placed immedi- 

 ately 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 dis- 

 tinct iris, with a kidney-shaped pupil. This eye is 

 remarkable for the total absence of a cornea ; the 

 integuments of the head being continued over the 

 iris, and reflected over the edges of the pupil, 

 giving a covering to the external surface of the 

 lens ; there is, of course, no chamber for containing 

 an aqueous humour. The globe of the eye is nearly 

 spherical ; but the sclerotica is double, leaving, at 



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

 Mediterranean, having its eyes supported on a long jointed tube, con- 

 sisting of two articulations, which enables the animal to move them 

 in various directions, like the arms of a telegraph. 



