18 BRITISH PARASITIC COPEPODA. 



shown by Hesse and others to be only the cyclops stage 

 of the Caligoida. The cyclops form grows, and the 

 various appendages make their appearance in regular 

 order. When the appendages are developed the 

 filament separates at its junction with the frontal 

 margin, and the parasite is free to move about over its 

 host or to seek a new one. A notch is left in the 

 frontal margin when the filament separates, and con- 

 tinues all through the adult life. The male, at the 

 conclusion of its cyclops stage, is practically fully de- 

 veloped. The female remains in an immature con- 

 dition until it is fertilized and the ova begin to pass 

 down the oviducts. The genital segment then in- 

 creases in size to its normal adult condition. 



Tribe Lehkeoida. 



The tjrpical member of this tribe is Lemsea bran- 

 chialis Linn. It presents one of the most marked 

 examples of retrograde development which is to be 

 found in the whole group of parasitic Copepoda. It 

 is difficult to believe at a first glance that the parasite 

 is a crustacean and that it is related to Lepeophtheirus. 

 There is great excuse for the difficulty experienced by 

 the earlier zoologists in deciding the true position of 

 the members of this family in the animal kingdom. 

 Nothing was then known about their life-history, and 

 it is not surprising that Linnaeus included Lemma 

 amongst his Vermes. 



The adult female is found on the gills of various 

 Gadoids, such as cod, haddock, and whiting. (See 

 Plate A.) Immature (cyclops stage) males, and 

 females with adult males attached, are to be found on 

 the apex of the gill-filaments of flounders and plaice, 

 sometimes in large numbers. The adult female is 



