199 



4th pair, which is only composed of 2 joints. Last pair of legs with the free 

 joint well defined, bisetose. 



Remarks. This genus, the type of the present family, was established 

 by Nordman as early as the year 1832, to include a peculiar parasite (E. 

 Sieboldi) found by him on the gills of various fresh-water fishes. Another 

 species apparently referable to this genus was subsequently recorded by Van 

 Beneden under the name of E. nanus. Only the type species is as yet 

 represented in the fauna of Norway. 



106. Ergasilus Sieboldi, Nordman. 

 (PI. CXI). 



Ergasilus Sieboldi, Nordman, Mikrographische Beitrage zur Naturgeschichte wirbelloser Thiere, 



Heft 2. 



Specific Characters. Young female (before affixion). Body moderately 

 slender and gradually tapered behind, with the anterior division oblong in 

 outline, greatest width not nearly attaining half the length and occurring in 

 front of the middle. Cephalon well defined from the trunk and almost penta- 

 gonal in form, the frontal part being somewhat produced and narrowly rounded. 

 Trunkal segments defined from each other by deep lateral incisions and gradu- 

 ally diminishing in size, epimeral plates obtusely rounded; last segment very 

 small, but well defined. Tail comparatively short, scarcely exceeding in length 

 J /4 of the anterior division; genital segment somewhat dilated and equal in 

 length to the remaining 3 segments combined. Caudal rami narrow, sub-linear 

 in form, and scarcely at all divergent, equalling about in length the last 2 

 segments combined; outer edge quite smooth, apex carrying 3 very unequal 

 setae the inner one much the largest, exceeding in length the tail. Eye very 

 conspicuous with dark pigment. Anterior antennae scarcely exceeding half the 

 length of the cephalon, and composed of 6 well defined joints clothed with 

 slender diverging setae; last joint the smallest, the others not very different in 

 size. Posterior antennae exceedingly large and powerful, and almost quite nacked; 

 basal joint thick and massive, forming with the middle joint a geniculate bend; 

 the latter joint much elongated and sligtly narrowed distally, dactylar joint 

 cylindrical in form and terminating in t a very strong curved claw. Natatory legs 

 with the basal part rather broad, rami comparatively short and nearly equal- 

 sized; spines of the outer ramus very small and quite wanting on the middle 

 joint in 2nd and 3rd pairs; terminal joint of both rami short, lamelliform, with 

 densely crowded marginal setae. Last pair of legs with the free joint oblong 

 oval in form ; apical setae comparatively short. 



