EYE 281 



as they are adapted in a remarkable manner both as regards position 

 and structure to the special conditions under which these Fishes 

 live in comparative darkness. Inmost Fishes, the laterally situated 

 eyes are capable of monocular vision only, while most deep-sea forms 

 possess binocular " telescopic " eyes, the axis of the two organs being 

 nearly parallel. The eye-ball, moreover, is much elongated (Fig 206), 

 so that there is a considerable distance between its inner wall and 

 the lens ; the latter is especially large, and the cornea is very 

 convex and forms a considerable part of the wall of the eye-ball ; 

 the iris is almost entirely wanting ; and finally, the retina is differ- 

 entiated into a main portion and a small accessory portion usually 

 situated only on that part of the wall which is nearest to the 

 median line of the head. The eye-muscles are more or less reduced 

 and shifted in position, while the apparatus for accommodation, the 

 suspensory ligament and retractor of the lens, are well developed. 

 These modifications result in allowing as many rays of light as 

 possible to enter the eye and spread out over the retina. 1 



In the Flat-fishes (Pleuronectidie), which have acquired the 

 habit of swimming and lying on the bottom on one side, the eye of 

 the lower side gradually rotates so as to reach the upper surface. 



In Dipnoans the eye is relatively small as compared with that 

 of other Fishes. In Protopterus the sclerotic is partly cartilaginous, 

 and the lens is globular and relatively large. As already mentioned, 

 there is apparently no processus falciformis,and a tape t urn, argentea, 

 and " choroid gland " are wanting. On the whole, the eye is inter- 

 mediate in structure between that of Ganoids and that of Urodeles. 



Amphibians. The eyes of Amphibians are in general 

 relatively small, and do not exhibit any essential advance in 

 structure as compared with those of Fishes : in certain respects 

 they show negative characters as compared with the latter, for an 

 argentea, a tapetum, a " choroid gland," and a processus falciformis 

 and campanula Halleri are wanting. 



The eye-ball is nearly globular and the cornea moderately 

 convex ; the pupil is round, or occasionally three-cornered (Bom- 

 binator). The large lens is more convex on its inner than on its 

 outer surface, especially in Anurans, and the larvae usually have a 

 smaller lens-index than adults. Accommodation does not take place 

 by an alteration in form of the lens, but in some cases, apparently, 

 the lens can be shifted towards the cornea by the action of the 

 ciliary muscle.- Ciliary processes can be recognised in Urodeles, 

 but are much more distinct in Anurans, and the iris has well- 

 developed smooth muscles. The sclerotic, as in Fishes, encloses 



1 In Periophthalmus and Boleophthalmus the structure of the eye is pecu- 

 liarly modified for vision in the air as well as in the water. 



A protractor of the lens, consisting of smooth fibres, occurs amongst 

 Urodeles and in the Frog, the fibres, however, taking a different course in the 

 two cases. It is not known whether this muscle is the homologueof the retractor 

 of the lens in Fishes. 



