348 THE SENSORIAL FUNCTIONS. 



ever, Is not the case with the Entomostraca^ comprising 

 the various species of Monocidi, in which the two eyes are 

 brought so close to one another, as apparently to constitute 

 a sini!;lc 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 moviiTg 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 tlie latter in the eye of the Cephalopoda; 

 for Sepix 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 higlily 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 tlic 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 lea Mollusques; Memoir sur Ic Poulpe, p. 37. In the 

 Octopus there are folds of the skin, which appear to be nutiments of eye-lids. 



