848 THE SENSORIAL FUNCTIONS. 



ever, 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 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 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 Sepias differ from all the tribes belonging to the inferior 

 orders of mollusca in having large and efficient eyes, con- 

 taining a hemispherical vitreous humour, placed immediate- 

 ly before a concave retina, and receiving in front a large 

 and highly convex crystalline lens, which is soft at its exte- 

 rior, but rapidly increases in density, and contains a nucleus 

 of great hardness: there is also a pigmentum nigrum, and a 

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

 markable 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 sur- 

 face of the lens: there is, of course, no chamber for contain- 

 ing an aqueous humour. The globe of the eye is nearly 

 spherical, but the sclerotica is double, leaving, at the poste- 

 rior part, between its two portions, a considerable space, oc- 

 cupied by the large ganglion of the optic nerve, with its nu- 

 merous filaments, which are embedded in a soft glandular 

 substance.* 



The eyes of Fishes differ from those of sepiae principally 

 in the addition of a distinct cornea, exterior to the lens and 

 iris, but having only a slight degree of convexity. This, 

 indeed, is the case with all aquatic animals; for, since the 

 difference of density between the cornea and the external 

 medium is but small, the refractive power of any cornea, 



* See Cuvier, sur les Mollusques; Memoir sur le Poulpe, p. 37. In the 

 Octopus there are folds of the skin, which appear to be rudiments of eye-lidr 



