REARING THE YOUNG FRY. 203 



The emaciation continues, the blackness of the skin 

 increases ; the fish finally becomes totally blind and 

 dies. I know of no cause or remedy, though I 

 have noticed that more cases occur where the water 

 has become somewhat foul, and once I thought a fish 

 affected with this disorder recovered on being removed 

 into better water, but I do not feel certain of it. The 

 disease attacks young and old alike, and is not conta- 

 gious. 



14. Irritation of the optic nerve. Fishes, as is well 

 known, have no eyelids to protect their eyes from 

 excessive light. It is therefore a very serious thing to 

 young fry, that have been used only to the dark, to be 

 suddenly exposed to the glare of the sun ; and it some- 

 times happens that when they are so exposed, and 

 cannot escape from the sunlight, their brains become 

 hurt, they assume most unnatural positions and move- 

 ments, and after darting about frantically, like crazy 

 creatures, for a few moments, they die. I have sup- 

 posed that the unaccustomed light produces an irrita- 

 tion of the optic nerve, and have so named it. 



15. Liflammation of the gills. This corresponds to 

 inflammation of the lungs in animals, and it is the re- 

 sult usually of crowding too many trout into too small 

 a space, without a sufficient change of water. Their 

 gills or lungs have too much work to do, and this, with 

 breathing over the impure water, produces inflamma- 

 tion. It is a lingering disorder, more in that particular 

 like consumption in higher orders. The affected fish 

 may contrive to live for some time, and eat the same, 

 but will not grow any; they will become attenuated, and 



