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and very difficult or impossible to examine in detail on intact specimens, so that in numerous 
cases one is obliged to undertake a difficult dissection, and to place the head and the genital 
area in a preparation; lastly, the general form of the body in both sexes, and particularly 
in the females, is far less fixed than in the free-living forms. On account of these circum- 
stances it is sometimes difficult to form a positive judgement about some species, e. g. in 
how far they present varieties of one species, or form separate species. It is sufficiently 
well known that a similar difficulty is not unfrequent with regard to the free forms, and 
from what has been said about Choniostomatidae, it is easy to understand that, with respect 
to this family, the difficulties are sometimes so great that a final settlement of some questions 
must be left to the future. 
It has been specially mentioned that most species of Choniostomatidee have been 
found each on its particular species of Malacostraca, but, at the same time, I can prove to 
a certainty that the same species can be found on different species of the same genus (e. ¢. 
Choniostoma Hansenii on two species of Hippolyte), or even in forms of two different genera 
(Mysidion commune on Parerythrops and on two species of Eryfhrops); and further, on the same 
species of host one may find two species of parasites in the branchial cavity or in the mar- 
supium, nay even two species in the same marsupium (Mysidion commune and Mysid. abys- 
sorum in Er, abyssorum). ‘The result hereof is that we cannot absolutely take for granted 
that we know a parasite, because we have found it on a certain host, nor that a parasite 
belongs to an unknown species, because it is found in a host that is not mentioned in this 
work. All the same, in most cases the host is of the greatest importance in determining a 
parasite, and where parasites are found in new hosts, most frequently they will prove them- 
selves to be new species. 
Most of the species established in this work have been easy to distinguish from each 
other, and in the majority of cases there has been no hesitation at all in establishing the 
different species. It is mentioned above that on Perioculodes longimanus (Sp. Bate) I found 
females which were exactly like the Spheronella paradoxa living on species of Bathyporeia 
Lindstr., but as the male belonging to the females found on Perioculodes is wanting, I have 
not been able to decide whether the same species really lives on forms of different families. 
On account of rather small material, [ have also had a little doubt concerning the identity 
of the forms found on Diastylis cornuta Boeck and D. levis Norm.; but with regard to this 
question, as well as to Aspidoecia Normani, I refer to the subsequent special representation. 
The greatest difficulty | met with in the species very closely allied to Spheronella Leuckartii 
Sal. Of these species I have established eight, taken in six genera belonging to four 
different families of Amphipoda, and four of these species of hosts (belonging to four different 
families) came from Denmark, two from Sicily, one from the West-Indies, one from Hong- 
Kong. The difficulties were so great, that I hesitated for a long time whether to establish 
them each separately, or as belonging all to one species. Though this question will be 
treated more thoroughly in the systematic part, I thought it right to call attention to it here. 
