34 ORGANS OF SENSE. 



one another at the edges of its free prolongation, which forms 

 the iris, and between these two layers occur longitudinal mus- 

 cular fibres. A layer of cartilage underlying the tapetnni. forms 

 the inner ocular capsule, which extends externally as far as the 

 iris, and is penetrated on its inner side by the fibres of the optic 

 nerve. The ciliary body is formed of connective tissue with 

 muscular fibres and is placed, as a thick rim, upon the free edire 

 of the inner capsule. The lens is composed of layers of struc- 

 tureless membrane, which are cuticular productions of the ciliary 

 body ; it is almost cylindrically elongated in the direction of the 

 axis of the eye. The vitreous humor is a transparent fluid. The 

 retina, lining the inner capsule, has an outer and an inner 

 stratum, separated by a pigment layer. The inner stratum is 

 composed of prismatic or cylindrical rods, the inner ends of 

 which, turned towards the ocular cavity, are covered by a mem- 

 brane ; the outer stratum is filled with the plexus of the optic 

 nerve-fibres and with glanglionic cells, connected by tissue ; thus 

 the nerve terminations must penetrate the pigment lying between 

 the two strata of the retina in order to reach the rods in the 

 inner stratum. 



The pedunculated eye of the Nautilus is much simplified in 

 its structure, having neither cornea, lens nor vitreous humor. 

 The creeping habits of the animal, the abundant protection 

 afforded by its external shell, its want of offensive armor upon 

 the tentacles are here found correlative with a degradation of the 

 visual organ. 



A great difference in the size of the eyes in the pelagic and 

 littoral genera accompanies the difference of habit. The littoral 

 Octopus, always existing where the light of the sun penetrates 

 with more or less power, has small eyes, whilst they are. enormous 

 in those gene rn which inhabit the high sens, peiietrat iiio- i<> givat 

 depths, and which are evidently nocturnal in their h:bits. So 

 also we find the situation of the eyes to differ according to the 

 habits of the cephalopods: thus the shore species, espccwllv the 

 creepers, have their eyes placed laterally ou the liack of the head, 

 that they may look above and around, but not below t hem. whilst 

 the swimmers on the contrary have their large eves placed 

 directly on the sides of the head to give 1 hem equal visual powers 

 in all directions. The eyes in the octopods are fixed, without 



