34 ORCiANS OF SENSE. 



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

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

 cular fibres. A layer of cartilage underlying the tapetum, 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 hodij is formed of connective tissue with 

 muscular fibres and is i)laced, as a thick rim, upon the free edge 

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

 tureless membrane, whicli 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. Tlie 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 ortler to reach the rods in the 

 inner stratum. 



The pedunculated eye of the Nautilus is much sim[)lified 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 ofi'ensive armor udou 

 the tentacles are here found correlative with a degradation of the 

 visual organ. 



A great ditference in the size of the eyes in tiie |>elagic and 

 littoral genera accomi)anies the ditference of habit. The littoral 

 Octopus, always existing where the liglit of the sun penetrates 

 with more or less power, has small eyes, whilst the}' are enormous 

 in those genera which inhabit the high seas, penetrating to great 

 depths, and which are evidently nocturnal in their habits. So 

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

 habits of the cephaloj^ods : thus the shore species, especially the 

 creepers, have their eyes placed laterally on the back of the head, 

 that they may look above and around, but not Itelovv them, whilst 

 the swimmers on the contrary have their large eyes placed 

 directly on the sides of tiie head to give them equal visual powers 

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



