2 BRITISH PARASITIC COPEPODA. 



the gill-covers ; sometimes they may be found clinging 

 to the roof and sides of the mouth, as well as on the 

 tongue, especially the under side of it, and also in the 

 nasal fossae. At other times they make excavations in 

 the flesh of the fish, causing sores of a more or less 

 serious nature, and they may even penetrate into 

 the abdominal cavity, as occasionally happens with 

 Lernaean parasites. 



The degree of parasitism varies greatly even among 

 closely allied species. In somes cases the relationship 

 of the crustacean to the fish is decidedly that of a 

 parasite, while in others it would scarcely be accurate 

 to describe the position of the crustacean to the fish 

 as truly parasitic. Sometimes, however, it is more 

 convenient to use the terms " parasite" or " parasitic " 

 in the wider sense to avoid any confusion which might 

 arise by trying to define in each particular case the 

 degree of relationship of the one to the other, and it is 

 in the wider sense that these terms are used here. 



The study of the Parasitic Crustacea is in some 

 respects more difficult, if also more interesting, than 

 that of the species which live under normal conditions. 

 Their structure has become more or less altered by 

 reason of their parasitic habits, and the forms which 

 some of them assume are greatly at variance with 

 those of free-living species. The forms of some of 

 these parasites are so abnormal and grotesque that 

 even experienced investigators have been deceived, and 

 have failed to recognize their relationship to the 

 Crustacea, and only by the study of their life-histories 

 have their true affinities been determined. 



Among the crustacean parasites of fishes the Cope- 

 poda are probably more numerously represented than 

 any other of the crustacean Orders, and what we pro- 



