ORGANS OF THE SENSES* 265 



the coats of which they are composed. Their eye-lids are 

 still as rudimentary as in the cephalopods, and the liquid 

 element around them is their only lacrymal as well as their 

 salivary fluid. The pupil is large in fishes and the iris is 

 almost motionless. The outer layer of the choroid passes 

 with its shining pearly lustre over the front of the iris, and 

 the dark inner layer lines the posterior surface of the uvea. 

 This inner layer forms a kind of pecten, passing obliquely 

 forwards to- the lens, but without the pigment which covers 

 it in birds. The ciliary ligament is always present, but the 

 ciliary processes are rarely developed, and the foramen 

 centrale is not perceptible in the axis of vision. The eyes 

 are generally quite lateral in their direction, and notwith- 

 standing the great irregularity of their outward form, the 

 sphericity of the retina is generally preserved by the glandu- 

 lar and adipose substances interposed behind between the 

 layers of the choroid coat. The sclerotic is easily perceived 

 to be continuous with the sheath of the optic nerve and the 

 dura mater, and sometimes, as in the xiphias, the consoli- 

 dated portion of the sclerotic envelopes the ball of the eye 

 like an osseous band, with a thickened anterior round margin 

 to receive the circumference of the cornea and to support the 

 periphery of the iris. The fatty substance commonly depo- 

 sited between the layers of the choroid at the back of the eye 

 in fishes is traversed by numerous branches from the ocular 

 artery, and by the ciliary veins returning to the choroid 

 gland. The inner of the two layers of the choroid is often 

 distinctly separable into two, an exterior formed by the 

 straight parallel ciliary veins, and an interior, formed by the 

 ramifications of the ciliary arteries, on which the pigment 

 rests. The optic nerve (104. A. g), generally presents a con- 

 tracted appearance at the place where it penetrates the vas- 

 cular layer of the choroid (104. A. k) 9 and expands into a soft 

 and pulpy retina which terminates by an even margin near 

 the base of the uvea. The organs of vision are smallest in 

 anguilliform fishes and such as burrow in the mud or sand ; 

 they are larger in predaceous fishes which frequent the dark 

 depths of the wide ocean than in those which reside on the 

 shallow coasts or in fresh waters ; their mobility is encreased 

 in the active muscular plagiostome fishes by being supported 

 on cartilaginous peduncles ; they are moved in fishes by 



