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KATHLEEX H ADDON. 
tissue, recalling in its structure and nutritive function tlie 
root system of the parasitic Rhiz()cepliala amongst the Cirri- 
pedes. It seems hardly likely that an endoparasitic stage 
is included in its life-history, but of this we are entirely 
ignorant. The adult males have a sac-like body, which 
remains enclosed in the last larval skin; they live attached 
to the l)ody of the female in the region of the genital 
apertures. 
8teenstrnp and Lhtken ( 12), who first described this 
species, Herpyllobius arcticus, found it parasitic on 
certain polychmte worms from Greenland, and recognised 
that the body consisted of two parts, the external bod}" and 
the part within the host, joined together by a thin stalk. 
The external body was rounded, and bore a pair of large 
egg-sacs, whilst the internal portion they believed to vary in 
shape according to the amount of space available within the 
worm. They found no males. 
Two years later Krdyer (7) published an account of a 
parasite, which he called Sileninm pol ynoes, found infecting 
Polynoe cirrata in Greenland. He only described the 
portion of the parasite external to its host, but he was more 
fortunate than Steenstrup and Liitken, in that lie found the 
males, four or five in number, attached near the egg-sacs of 
the female; they were minute Cyclops-like forms, about 
• 1-3 mm. long and half as broad, the anterior end of the body 
being drawn out and inserted finnly into the body of the 
female. He gave some details of the appearance of both 
male and female, which will be dealt with later in discussing 
the anatomy of these forms. 
In 1874 Claus ( 1 ) published a paper, in which he placed 
Silenium among the Lernaeopodida3, and suggested that it 
was identical with the Herpyllobius described by Steenstrup 
and Liitken. Further light was thrown on the systematic 
position of this animal by Levinsen (9), who, in an account 
of some parasitic copepods, gave a summary of the generic 
characters of Herpyllobius and the specific characters 
of the two species then known, H. arcticus (Stp. and Ltk.) 
