34 oi;<, \\> 01 BEH 



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

 the iris, and between tin-si- two layers occur longitudinal mus- 

 cular fibres. A layer of cart ilage underl ying tin- tapetum. forms 

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

 iris. an<l is penetrated on its inner side by the fibres of the optic 

 nerve. r riie riliunj Inxlij is formed of connect ive tissue with 

 muscular tilires ami is placed, as a thick rim, upon the free edge 

 of the inner capsule. The /<//.- is composed of layers of struc- 

 tureless memltraiie. which arc ciiticular productions of the ciliary 

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

 axis of the eye. The vitreous humor is a lr:insp:i.ivnt 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, art- covered by a mem- 

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

 nerve-fibres and with glanglionic cells, connected ly 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 peduncnlated 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 unon 

 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 dilference of habit. The littoral 

 Octopus, always existing where the light of the sun penetl 

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

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

 depths, and which are evidently nocturnal in their habits. >o 

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

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

 Creepers, have their eyes placed laterally on t he back of 1 he head, 

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

 the swimmers on the contrary have their large eyes placed 

 directly on the sides of the he.-id in give them etjnal visual p<>-. 

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



