I. INTRODUCTORY REMARKS. 



ri^he majority of the species here described I found in examining systematically for this 

 JL purpose the collections of the Zoological Museum in Copenhagen. Throughout a number 

 of years the two directors of the entomological department, the late Professor J. C. Schiodte 

 and his successor, Inspector Dr. F.Meinert, have taken care not only to acquire as many 

 species as possible, but — of the smaller forms — also as many individuals as could be 

 procured, so that of a good many northern Amphipoda, and of a great number of Danish 

 Amphipoda, Cumacea, etc., the museum possesses hundreds of specimens. This has been of 

 the greatest use to me in my researches, for while a few of the parasites — at least of 

 those found on our own material — are met with rather frequently, the greater number are 

 very rare, and a considerable part so scarce, that only one or two specimens are found on 

 each hundred of the animals examined. As a matter of course, I have examined numerous 

 species without finding a single parasite. 



Of the following forty-three species only one lives on the outside of its host (Mysidse 

 verse), two occur in the branchial cavity of Cumacea, two in the branchial cavity of Hippolyte ; 

 all the remaining species are only found in the marsupium of the female of Amphipoda 

 Gammaridea, Isopoda, Cumacea and Mysidse verse (or sometimes in young individuals of 

 Amphipoda on the ventral side of the thorax between the gills). In the Isopoda, the 

 Mysidse, and sometimes in the Amphipoda, parasites can be seen by looking through the 

 plates of the marsupium. In most Amphipoda and in Cumacea the marsupium has to be 

 submitted to- a closer examination; some of the plates have to be lifted up and examined 

 through a lens; in the small forms even the adult parasites can only be discovered by help 

 of a simple microscope. Where a closer search of an infested marsupium is, required, it is 

 usually necessary to place the host in a hollow ground glass -plate under water, and to 

 examine it very carefully twenty or thirty times magnified under a simple microscope, in 

 order to be able to discover the male animals which are generally 1 /i — 1 /e mm. in length, 

 as well as the free larvae and the pupse, and to find out the way in which these minute 

 animals are hinged. 



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