XXXIV THE EYE. 



, weever or sting-fish, Trachinus, has this modification of the upper -eyelid, 

 and so also has the tropical crocodile-fish, PlatycephaluSjia. the last of which, 

 however, very rudimentary scales .are also perceived, and the iris has two 

 tongue-like excrescences. 



This opaque upper lid, evidently.used as a protection, is found in the, 

 flat-fishes, Pleuronednlrr, while another modification may likewise be present. 

 If the eye of a turbot, Bhombus maximus, is. examined it. will be seen that 

 not only does thick opaque and coloured skin cover its upper and lateral 

 portions, but, being insuflicient to protect the eye from the irritation of the 

 sand wherewith it covers itself, it is able to raise a thick lower eyelid, or else 

 to depress the transparent portion of the globe of the eye below this fold of 

 the skin. In the sun-fish, Orthagoriscus, as previously observed, we find a 

 circular palpebral fold, provided with a sphincter muscle. 



Passing on to the rays and skates, which are sand-loving forms, we still 

 see immovable and opaque skin covering the upper portion of the eyeball ; 

 but, irrespective of this protective modification, there are other means which 

 can be used in order to prevent too much light obtaining entrance through 

 the pnpil. This is effected by certain veils or jorocesses, qf different forms, 

 which can be employed to cut off any excess of light. I have already 

 alluded to the teleostean crocodile-fish, Platycephalus, which lives in the 

 ti'opics, and resides at considerable depths, but often has to ascend to near 

 the surface in order to obtain its food. Here we find two processes of the 

 iris which can be employed as curtains — the upper and larger exists along 

 the upper margin of the pupil, and the smaller along the lower. So far as 

 I have observed, it is in these ground-feeders which occasionally ascend 

 towards the surface, from a dull into a bright light, that the upper veil or flap 

 of the iris is developed, while in some a lower veil may likewise co-exist. The 

 same modification is perceived in many of the Elasmobranchs ; thus in the 

 homelyn-ray, Bala mactdata, we see on looking through the transparent 

 cornea that the pupil is not circular, but possesses a veil that has been aptly 

 compared to a vine-leaf (vol. ii, p. 334), provided with twelve digitations, that 

 can be let down and thus cut off some or all the rays of light. Muscular 

 fibres, which anastomose with each other, pass downwards aiong this flap, 

 and into each of the digitati9iis which descend from it. 



The lens itself may likewise be modified, as in the Anableps, the eye of 

 which is divided externally into two unequal portions by a sort of band of 

 opaque skin, which crosses its transparent cornea from one side to the other. 

 This fish, which usually swims with its head partially out of the water, thus 

 shows an anomaly that at fitst seems very puzzling, for why should it be 

 furnished with an opaque band precluding any rays of light entering the 

 orbit along a central horizontal line ? On looking through the cornea it 

 appears as if this opaque band divided two pupils, preventing any rays 



