PREFACE. 
his work contains an account of forty-three species of Copepoda, all parasitic on mala- 
. Crustacea, and all belonging to the same family. When in 1890 I began 
my study of this group, there were published descriptions of only three species, and mention 
had been made of a fourth. Two more have been since described and a seventh named, 
but not described; so that until now (July 1897) only five species have been reaJly made 
known. In the present work I increase this number about nine times, and yet, most 
likely, my discoveries only extend to one fifth or one sixth or perhaps a much smaller part 
of the species extant. I have been brought to this conclusion by the consideration that no 
less than thirty-three of my species have been found exclusively on Crustacea in the 
Zoological Museum of the Copenhagen University. What multitudes of these animals are 
likely to be discovered, when some day the large foreign museums acquire rich collections 
of non-decapod Malacostraca, and when this material is submitted to a thorough research! 
On the whole, my studies of late years have given me the impression that of nearly all the 
Crustacea living on the bottom of the sea — the Decapods excepted — we only know 
from about half down to a very small percentage of existing species. Especially to the 
parasitic forms does this apply, and I think one of the most important results of the present 
work is to show the wealth of a group, which hitherto has occupied only a very diminutive 
place. It may be added that, in the course of the last two years, I have found on the 
material brought home from the sea near Iceland and Greenland by the »Ingolf« expedition 
several new forms which cannot be included in the present treatise, but which will be 
subjected to future examination. 
A chance led me to this study. In dissecting a female of Jdothea marina (L.) I 
discovered in its marsupium an unknown parasite belonging to the Epicaridea, and further 
researches led to the discovery of a number of specimens of this species and of a form 
nearly akin to it on Edotia nodulosa (Ky.). Both parasites were afterwards described from 
