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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 Choniostomatidae 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. g. 

 Choniostoma Hansenii on two species of Hippolyte), or even in forms of two different genera 

 (Mysidion commune on Parerythrpps and on two species of Erythrops) ; 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 Ave 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 tins 

 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 Sphceronella 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, I have also had a little doubt concerning the identity 

 of the forms found on Diastylis cornuta Boeck and D. Icevis Norm. ; but with regard to this 

 question, as well as to Aspidoecia Normani, I refer to the subsequent special representation. 

 The greatest difficulty I met with in the species very closely allied to Sphceronella 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. 



