16 OROANS OF SENSE, 



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

 iris, and between these two la^^ers occur longitudinal muscular 

 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 body is formed of connective tissue with 

 muscular fibres and is placed, as a thick rim, npon the free edge 

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

 less membrane, which are cuticular productions of the ciliary 

 body ; it is almost cjdindrically 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 cavit}^, are covered by a 

 membrane ; the outer stratum is filled with the plexus of the 

 optic nerve fibres and with ganglionic 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 aflTorded 

 by its external shell, its want of ofl"ensive armor upon the 

 tentacles are here found correlative with a degradation of the 

 visual organ. 



A great difierence 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 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 cephalopods : thus the shore species, especially the 

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

 that they maj^ 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 head to give them equal visual 

 powers in all directions. The eyes in the octopods are fixed, 

 without movement, whilst in the decapods they are free and 

 capable of movement ; in the former the skin is susceptible of 

 contraction, so as to cover the eye entirely, fulfilling the 

 functions of an eyelid ; whilst in the latter the littoral species 

 are furnished with this protection, but the pelagic ones are 

 without it. 



Gastropoda. We have already seen that the eyes are variouslj'' 

 situated upon or branching from the tentacles {ommatophores) 



