GREA T BRIT A IN. 25 



not very extensive. Its centre is perforated by the pupil, 

 for the transmission of light to the interior of the eye. 

 The pupil is mostly circular, but in some forms is vertically 

 or horizontally elliptical, as in the majority of sharks, or it 

 may even be almost quadrangular, as in Galeus. Occasion- 

 ally a veil or flap descends from the edge of the iris at the 

 upper portion of the pupil, as may be seen in some pleuro- 

 nectoids, as in the turbot, or in a few of the rays of our 

 seas, as the Homelyn, wherein this process is somewhat 

 similar to the shape of a vine-leaf, surrounded by twelve 

 short projections. These forms live more or less at the 

 bottom of the sea, and to them a large amount of light is 

 evidently unsuited ; they are consequently provided with 

 an extensible and retractile veil, capable of regulating, or 

 even entirely preventing rays entering from above. 



The aqueous humour in the anterior chamber is very 

 small in amount, while the vitreous body which fills up the 

 greater portion of the ball of the eye is of firmer consistence 

 than in other vertebrate animals. The crystalline lens is 

 nearly or quite spherical. 



Fishes' eyes present a great variety as well as extent of 

 outer coverings, some portions of which are opaque and 

 others transparent. The skin which covers the eye is 

 commonly thin in substance and transparent in character, 

 being modified in order to suit altered conditions. In some 

 forms, as in the mackerel, what are usually termed " adipose 

 lids " are present on either side of the eye, and covering a 

 portion of its ball. These lids are formed by a single layer 

 of skin having been continued from contiguous parts of the 

 head, over the front of the eye, but bulging anteriorly and 

 posteriorly ; these bulging on loose portions become a 

 double fold, between the layers of which fat may be found. 

 Or a double layer of skin may pass entirely across the eye, 



