BRITISH PARASITIC COPEPODA. 



INTRODUCTION. 



PARASITIC habits are not peculiar to any class of 

 organisms, but are met with in all departments of life, 

 vegetable as well as animal. Nevertheless there are 

 certain groups both of plants and animals whose en- 

 vironment seems to be specially favourable for the 

 adoption of habits of a parasitic or semi-parasitic 

 kind. Such habits seem to prevail to a considerable 

 extent among the Crustacea, and particularly among 

 the so-called lower forms belonging to that class. 

 Many kinds of animals are required to act as hosts for 

 these crustaceans, and curiously enough not a few of 

 them belong to the same class as that to which the 

 parasites themselves belong. Many fishes are also 

 infested by these crustacean parasites. Fishes live in 

 a medium where on every side they are exposed to the 

 attacks of all kinds of parasitic forms, so that very 

 few of the fishes examined by us are found to be 

 entirely free from these organisms. 



At a fish-market where quantities of fishes of various 

 kinds are being landed, crustacean parasites may be 

 frequently observed on the skin of the fishes, or found 

 adhering to their fins, their gills, or on the inside of 



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