200 DOMESTICATED TROUT. 



swells and grows soft, and an ulcer appears on the 

 top of the skull, which discharges a thin, watery fluid 

 when punctured. It is not contagious, but always 

 fatal. The remedy is found in prevention. It is to 

 keep the water pure, and give the trout plenty of 

 earth. 



ii. Animal parasites * This is a very alarming and 

 . destructive cause of death among the young fry, and 

 all the more because the parasite attacks the best 

 and fattest and healthiest fish. They come suddenly 

 and unexpectedly, sometimes as early as the ist of 

 May, and first show themselves as a little bunch of 

 whitish jelly-like matter on the back or sides of the 

 fish, in most cases not far from the dorsal fin. At 

 first the fish does not appear to mind it much, and 

 feeds and remains in good condition for a day or two. 

 But soon after he seeks an eddy where the water is 

 still, refuses food, and dies within a week. This dis- 

 ease is fatal, and whether contagious or not, it is cer- 

 tain that whole boxes are attacked at once, and in the 

 instances within my experience every fish was de- 

 stroyed in ten days, none escaped ; it is the most 

 fatal and insidious disorder that I have encountered 

 in raising young fry. The microscope which I used 

 for examination revealed nothing but a gelatinous 

 protuberance on the body of the fish. I have sup- 

 posed it to be the eggs of some water insect floating 

 in the water, but provided with the power of attaching 

 itself to whatever it fell upon, like the eggs of perch 



* See Appendix I. for account of another class of animal 

 parasites, not discovered when this chapter was written. 



