144 ON SPECIES OF OSTRACODA FOUND 
traca,” issued by the Ray Society, and to the many valuable 
memoirs, which since the publication of that work have been 
added to the literature of the subject by Continental Naturalists; 
among these I would especially direct attention to the valuable 
monograph recently published by Dr. Leydig, “ Naturgeschichte 
der Daphniden.” 
The variations of external form among the Entomostraca, and 
especially in the parasitic Copepoda, are so great, and the functions 
which the different organs frequently assume are so curious and 
abnormal, that the elucidation of the identity of homological 
parts becomes a matter of no little difficulty, and of proportion- 
ably increased interest. In the functions attendant upon the act 
of generation we find the most remarkable phenomena; thus 
among the Cladocera we have some of the best established in- 
stances of true Parthenogenesis, and also a twofold mode of 
reproduction, the one by the ordinary eggs, the other by the 
ephippical or winter eggs; among the free Copepoda the act of 
impregnation is in many genera effected by the attachment of the 
strange “sperm tubes” to the body of the female, while among 
the parasitic tribe of Lernwordea the males are so minute in pro- 
portion to the females, and altogether so different in aspect that 
it has been a matter of dispute whether they were not larval 
forms. These extraordinary males, moreover, live attached to 
the female through life, or at any rate they have always hitherto 
been met with clinging to some portion of the body of the 
gentler sex. Then again there is in this sub-class a remarkable 
tenacity of life by which the eggs are enabled to survive the 
most intense cold of winter when completely frozen in blocks of 
solid ice, and also the drought of summer when the ponds, which 
they inhabit, are completely dried up. ‘The peculiar transpa- 
rency of a large number of the species offers to the physiologist 
the most unusual facilities for the study of the minute structure 
of the several systems which discharge the functions of nutrition, 
circulation, reproduction, and sensation. ~ Lastly, the peculiarities 
observable in the metamorphosis, habits and economy of the 
Entomostraca, their wonderful productiveness, and the part which 
from their extraordinary members they perform in cleansing water, 
