VISION. XXX111 



the case were the posterior lid overlapping the anterior one. These adipose 

 lids are formed by a single layer of skin having been continued from the 

 contiguous parts of the head, over the front of the eye, but bulging anteriorly 

 and posteriorly; these bulging -or loose portions become a double fold, 

 between the layers of which fat may be found. Sars, who paid particular 

 attention to the character of these lids, as seen in the living fish, remarked 

 that in the earlier part of the year off Norway they are much more opaque 

 than they are later on, due to the fish in these earlier months beiug fatter, 

 and that it is not until after they have spawned that the fat becomes absorbed, 

 and as a consequence the lids are transparent. 



There are also certain classes wherein a double layer of the skin may 

 partially or entirely cross the front surface of the eye, among which I propose 

 to allude to the well-known " blind " or " bib," Gad us licscus. In this fish, 

 belonging to the cod family, the orbital layer of skin may be popularly 

 described as splitting into two layers, the innermost passing on to the eye 

 and becoming attached to its front surface, as I have already described ; but, 

 due to the skin being in two layers, a sort of hollow bag is present, covering 

 the front of the eye, and^ this the fish appears to be able to distend under 

 certain conditions. One of these conditions is on its being drawn up 

 from great depths in the sea, at which time {due to the decreased pressure 

 of the water) its gases expand and this cavity becomes " baloon-shaped," 

 or, should fluid be inside it, like a "bleb" or blister. The' same 

 appearance now sets in as described in the loose and baggy portion of 

 the single layer around the globe of the eye, with this addition that in the 

 "bib" the entire structure of the bag in front of the eye becomes of an opaque 

 or white sodden character, and this may be seen either in the living fish 

 or after its death. The mangoe-fish and other Polynemi of the tropical 

 seas have a thick layer of transparent skin passing entirely across the 

 eye, while in the common mullet (Mugil) a vertical and elliptical opening 

 is present. 



Not only have we a transparent skin covering the eye -as described, 

 but also we may have it only partially thus modified, or largely covered with 

 opaque substances. Thus in the snipe- fish, Centriscus, which has been 

 captured on the British coast, some minute scales are present on the outer 

 fourth of the skin covering the globe-of the eye, except in its anterior portion ; 

 here they would mostly serve for protection from direct injuries. Some 

 forms have a species of almost fixed eyelid along the upper portion of the 

 eye, consisting of thickened and coloured skin, which may or may not be 

 furnished with rudimentary scales. This is commonly perceived in such as 

 reside on or near the sand, and are mostly bottom-feeders, and is very 

 probably protective ; in these forms of fish the iris or pupil is sometimes 

 unlike the round or oval shape whicn is generally seen. As an example, the 



c 



