BLIND FISH. 



471 



fish are specially characterised by the structure of the eye, which is quite unique 

 among vertebrates, and from which they derive their name. In each eye the 

 integuments are divided into an upper and a lower moiety by a dark-coloured 

 transverse band in the outer layer ; the pupil being likewise bisected in the same 

 plane by means of a lobe projecting from each side of the iris. The scales are of 

 small or moderate size ; the dorsal and anal fins short, the latter being placed in 

 advance of the line of the former, and in the male (which in this genus is larger 

 than the female) modified into a long, thick, scaly organ, with an aperture at the 

 end. These fishes are the largest existing members of the whole family, growing 

 nearly to a foot in length. They are abundant in North Brazil and the Guianas, 

 where they frequent mud-banks on the coast and in the estuaries of the larger 





FEMALE AND MALE DOUBLE-EYE (J nat. size). 



rivers ; many of them being often left stranded by the retiring tide, where they 

 progress on the slime by a series of leaps. After birth the young are carried about 

 by the female in a thin-skinned sac divided by a partition, until they are suffici- 

 ently advanced to take care of themselves. When swimming, these fishes frequently 

 go on the surface with the eye half in and half out of the water ; and it is in 

 accordance with this habit that the eyes are divided, the upper portion being able 

 to see in the air, while the lower is adapted for vision under water. That such is 

 really the case, is proved by the structure of the lens of the eye. In terrestrial 

 animals the lens is lenticular, that is to say, of the shape of two watch-glasses put 

 edge to edge ; whereas in ordinary fishes, which have to see in such a dense medium 

 as water, the lens is spherical. Now in the double-eye that portion of the organ of 

 vision which is above the level of the water has the lens lenticular, while in that 

 portion which is below the water the lens is spherical. In Brazil the flesh of these 

 fish forms an article of consumption. 



