i86 A HISTORY OF FISHES 



brates the iris of the eye is capable of great contraction, and, 

 acting hke the diaphragm of a camera, regulates the amount 

 of light allowed to enter the eye. In fishes it generally surrounds 

 a rounded pupil, and has comparatively little power of 

 contraction. It may be brightly coloured, red, orange, black, 

 blue, or green. 



Land vertebrates are able to accommodate the eyes to vision 

 at varying distances (that is to say, to focus the eyes on objects 

 both near and far away) by altering the convexity of the lens 

 through the action of special muscles ; in fishes the same end is 

 accomplished by simply changing the position of the lens with 

 regard to the retina. The retina itself has an elaborate structure, 

 and is made up of numerous sensitive cells ; its function is to 

 set up appropriate nervous impulses when acted upon by the 

 rays of light focused upon it by the lens, and thus to convey 

 to the brain a picture of the object upon which the fish has 

 fixed its attention. It may be noted here that as the eyes of a 

 fish are placed on either side of the head, what is known as 

 monocular vision is the rule, a fish being incapable of focusing 

 both its eyes on the same object at one and the same time. 



Some of the accessory structures associated with the eyes of 

 higher animals are wanting. For example, no lachrymal 

 glands are developed, so that a fish cannot shed a tear, nor is 

 this necessary when the outer surface of the eyeball is kept 

 constantly clean and moist by the surrounding water. No 

 fishes possess true eyelids, the skin and integuments of the head 

 simply passing over the eye and becoming transparent as they 

 cross the orbit. At the most, these are represented by a few 

 folds of skin at the margins of the eye, capable of little if any 

 movement, and even when stretched to their fullest extent 

 leaving the greater part of the eye uncovered. In some fishes, 

 notably some of the Grey Mullets {Mugilidae) and Herrings 

 [Clupeidae) , the skin over the eyeball is thickened, and, although 

 still transparent, covers the greater part of its outer surface, 

 leaving a small aperture in the centre. Such forms are said to 

 have an adipose eyelid. Some Sharks {e.g. the Tope Eugaleus) 

 have a third eyelid known as the nictitating membrane at the 

 front corner of the eye, which is freely movable and can be 

 pulled down to cover the whole surface. In bottom-living 

 forms like the Rays and Flat-fishes, the upper part of the 

 pupil is covered by a thick dark lobe, often covered with 

 scales, forming an eflfective curtain to shut off the light from 

 above (Figs. 14B; 40). 



