22 BRITISH PARASITIC C0PEP0DA. 



to the filament by a thin broad chitinous plate and by 

 the claw-like antennas. The young parasite assumes 

 the cyclops form and continues its progressive develop- 

 ment. The various appendages and internal organs 

 make their appearance. The male reaches maturity 

 without altering from the cyclops form, and undergoes 

 no further change. A considerable lengthening of 

 the female genital segment accompanies the develop- 

 ment of the various appendages. Fertilization takes 

 place, and the young female severs its connection with 

 the gill-filament. It leads a pelagic life for a time, 

 and is occasionally found in surface-plankton. The 

 males, unless accidentally separated, remain on the 

 gill- filaments after the females have gone. 



The cyclops form of Lernasa appears to cause con- 

 siderable damage to the gills of its temporary host. 

 The whole of the apex of the gill-ray assumes a tumid 

 appearance, and a considerable number of the gill- 

 filaments disappear from both sides. (See Plate A.*) 

 When the fertilized female leaves the gills of the 

 flounder or plaice the appendages and internal organs 

 have practically completed their development. A pair 

 of eyes similar in structure to that of adult Lepeo- 

 phtheirus is present on the anterior region of the 

 cephalo-thorax. The appendages are represented by 

 one pair of four-jointed antennules, one pair of claw- 

 like antennae, one pair of mandibles which are not 

 enclosed in the mouth-tube, one pair of maxillae, two 

 pairs of maxillipeds, and five pairs of feet. The body 

 is divided into five distinct segments — the cephalo- 

 thorax, three thoracic segments, and one terminal 

 segment which represents the genital segment and 

 abdomen. The whole of the terminal segment is 



* See also Plate XLII, fig. 8. 



