SYSTEMATIC ARRANGEMENT. 



In this account of the Copepoda found parasitic on 

 British fishes, the arrangement outlined by Professor 

 Gr. 0. Sars in the introduction to vol. iv of his great 

 work on the ' Crustacea of Norway ' has, so far as it 

 concerns these parasitic species, been followed by us. 



Professor Sars arranges the Copepoda into seven 

 divisions or tribes, three of which are represented 

 below viz., the Cyclopoida, the Caligoida, and the 

 Lernaeoida. The Cyclopoida show considerable varia- 

 tion in regard to their habits and manner of life ; some 

 are entirely free-living (as Cyclops), others are asso- 

 ciated with different organisms either as commensals 

 or messmates, their commensalism tending in some 

 cases in the adult stage towards a more or less true 

 parasitic life. Several species belonging to this tribe 

 are associated with fishes. 



The Caligoida, which are almost all parasitic on 

 fishes, also exhibit considerable differences in their 

 habits, but although none of them can be accurately 

 described as free-living in the adult stage, some species, 

 such as Caligus rapax, possess considerable freedom of 

 movement, and have, in consequence, a tolerably wide 

 distribution. In the majority of cases, however, the 

 Caligoida, though not usually permanently fixed to 

 their host, are limited in their movements, and do not 

 apparently, to any extent, migrate from one fish to 

 another ; and not only are they more or less confined to 

 particular fishes but sometimes also to particular parts 

 of the fish, hence such names as C aligns zei and 

 Lepeophtheirus pectoralis. In the first case the parasite 

 is usually found only on the dory (Zeus faber), while 

 in the second it occurs chiefly on the pectoral fins 

 of flat fishes, such as plaice, flounders, and dabs. 



The Lernaeoida differ from the other two tribes or 

 divisions by the females in the adult stage being all 



