24 Fishery Board for Scotland. 



times extends down through the muscles to the vertebral column, 

 as in fig. 91, which represents a plaice 8 inches (20cm.) in length. 

 The skin was in parts thrown into wrinkles (wr.). This would 

 seem to indicate that the disease was extending below the integu- 

 ment. On the head the ulcer attacks the bones of the skull, and 

 often destroys the eyes. 



AVhat appears to be the same disease was described by Johnstone. 

 He ascribed the disease to a fungus which he found in the liver, 

 kidney and mesenteries of the fish. 



The diseased fishes, taken on one occasion from the pond at Bay 

 of Xigg, in most cases exhibited the disease on the head. The 

 head was inflamed and swollen, or partly eaten away. Some of the 

 fish were blind. Their bodily condition was not bad. Some were 

 very thin, others were plump. When the fish were opened the 

 viscera appeared normal. The liver and kidney of one fish were 

 examined with a view to finding if either organ was infected with 

 parasites ; no parasites were observed. 



Riddell and Alexander have recently studied this disease. They 

 consider it a bacterial disease due to one of three bacilli which they 

 describe . 



Some sections of one of the ulcers have been prepared by Dr. F. 

 M. Milne, Dundee. Fig. 57 is a drawing made from one of the 

 sections. The portion (sk.) represents the disorganised skin, in 

 which the layers of the derma can be made out. The superficial 

 muscle layer is indicated at sm., while m. is a portion of the deeper 

 muscle layer. On the outside of the skin a little black pigment is 

 visible ; and a small quantity is also to be made out on the outer 

 side of the superficial muscle layer. The skin mass is covered 

 with nucleated cells. The muscle bundles next the skin show 

 vacuoles in them (fig. 55). All the muscles in the layer m. are 

 affected. The layer is about three times as thick as the superficial 

 muscle layer. A layer of connective tissue separates it from the 

 still deeper muscles. The latter, with the exception of one or two 

 bundles next the la}~er above, were normal (fig. 61) i.e., without 

 vacuoles. In the connective tissue laver a large vessel exhibited in 

 its wall a layer of black or brown pigment. 



The connected tissue between the vacuolated bundles is charged 

 with blood corpuscles. The vessels are shrunken, and the cor- 

 puscles project fi'om them (fig. 61). 



There were large vessels in the superficial la) T er, i.e., between 

 the muscle bundles : they had blood corpuscles thickly dotted along 

 them. The muscles themselves are evidently partly disorganised : 

 vacuoles were seen in some of them. 



A disease which had the form of rodent ulcers in the flesh of the 

 Brown Trout (Salvelinus fontinalis) is described by Calkins. It 

 was caused by Lympho-sporidium Truttae. The sporozoites develop 

 from a minute size to adult specimens (025-03 mm. in length) in 

 the lymph spaces round the intestine. The adults pass into the 

 muscle bundles about the intestine. Spore-forming individuals 

 (025-03 mm.) are found in the lymph surrounding the various 

 organs, and are quite numerous in the cavity of the gall-bladder and 

 of the intestine. The spores measure •002--003 mm. 



The cause of the disease in the Plaice is not yet elucidated. A 

 combination of causes is no doubt to be reckoned with. A bruise 



