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any one of the sixteen species known to infest the salmon is 

 capable of inflicting injuries upon this valuable fish. Mere 

 size of any given entozoon affords no criterion of its power 

 for mischief. Amongst quadrupeds even the largest hosts 

 succumb to comparatively small helminths. We see this 

 in the case of flukes causing rot not only in sheep, cattle, 

 and deer, but also amongst elephants. In all such cases 

 death is primarily due to over-crowding. As in our big 

 cities overcrowding causes the territory to suffer, so 

 likewise the passages, ducts, and channels of any vital 

 organ of the animal " host " suffer from the multiplication 

 of parasitic residents. Amongst fish, flukes work little 

 harm, since they rarely occupy any vital organ, nevertheless 

 with cetaceans the case is far otherwise. Mammals adapted 

 to enjoy an aquatic existence, and having an organisation 

 otherwise conforming to that of quadrupeds, do not escape 

 injury from flukes. To what extent they suffer is another 

 question. I have dissected a porpoise whose liver ducts 

 were extensively diseased, precisely as in cases of rot 

 occurring in their mammalian brethren of terrestrial habits. 

 The new species of fluke which I thus discovered in a 

 Firth of Forth cetacean has since been found by Dr. 

 Anderson in the Dolphin of the Ganges.* This small 

 parasite occurs in prodigious numbers. 



Save in exceptional instances, overcrowding, as before 

 remarked, lies at the root of all injury to the host, be it 

 bird, beast, or fish. With fish the tapeworms are most 

 destructive. Much depends upon the situation of the 

 parasite. When one finds the pancreatic cceca stuffed 

 with tapeworms, choking not only these appendages, but 



* Details are given in my paper on * Trematode parasites of the 

 Dolphins of the Ganges.' Journal of the Linnean Society for 1876. 

 Zool. Div., vol. xiii., p. 35 (with illustrations). 



