396 A HISTORY OF RECENT CRUSTACEA 
more than one host. This, if accepted, makes the distinc- 
tion of species particularly easy. It is, however, a rather 
wide generalisation. It appears to imply that the larval 
forms always settle on the same species as that occupied 
by their parents, or perish unless they do. Yet, if the 
larvee of a single brood were dispersed upon hosts of nearly 
related species, one might expect that to those placed in 
slightly novel circumstances, some difference of habit would 
result rather than destruction. Still the important point 
would remain that in each species of host the parasite is 
distinct in character, and in favour of their view that it 
is also distinct in species, Giard and Bonnier urge that 
‘often among closely related Epicarids there are consider- 
able physiological differences, sometimes even morpholo- 
gical differences relating solely to the male or the embryo, 
differences too important to be attributed simply to the 
difference of host.’ 
In the relations between the Epicaridea and their hosts 
a very singular circumstance has recently been brought to 
light. Rathke and other observers had commented on the 
unexplained peculiarity that the infested prawns and 
crabs all appeared to be females, and moreover sterile 
females. De Haan, as heretofore remarked, not unfre- 
quently records sterilé females of a different form from the 
fertile females of the same crab. Giard and Bonnier have 
shown that an infested female is not always absolutely 
sterile; but a few years earlier Professor Giard made the 
extremely interesting discovery that the parasites attack 
males just as freely as the other sex. Only, under the 
influence of the invader the distinguishing characters of 
the male are hindered from development, and a sort of 
intermediate appearance is permanently retained. 
Family 1.—Microniscide. 
This includes the least degraded forms, corresponding 
in general aspect to the second larval form of the other 
families. They are parasitic on Copepoda. 
Microniscus, Fritz Miiller, 1870, is the only genus. 
