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I. INTRODUCTORY REMARKS. 
he majority of the species here described I found in examining systematically for this 
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 (Mysida 
verze), 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 Myside vere (or sometimes in young individuals of 
Amphipoda on the ventral side of the thorax between the gills). In the Isopoda, the 
Myside, 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/:—1/s mm. in length, 
as well as the free larve and the pups, and to find out the way in which these minute 
animals are hinged. 
