Land and Fresh-Water Crustacea around Edinburgh. 375 
deposited in either one or two—but never more than two— 
little external sacs—the ovisacs—which are attached to the 
genital segment of the abdomen, and are in communication 
with the oviducts, so that the eggs, as they are successively 
matured within the body of the animal, pass into them. 
The ovisacs, especially when there are two of them, one on 
each side of the abdomen, as in Cyclops, enhance the grace- 
ful appearance and beauty of the little creatures, as they are 
usually more or less, and, occasionally, even brilliantly 
coloured. In the Cladocera the eggs are carried in an 
ovigerous chamber situated within the posterior portion of the 
sort of bivalve test with which many species are furnished, 
as for example in Daphnia, or within a dorsal sac formed 
by an outgrowth of the dorsal surface, as in Polyphemus or 
Bythotrephes. But whether the eggs are carried in ovisacs, 
as among the Copepoda, or in an ovigerous chamber, as 
among the Cladocera, they remain in these receptacles till 
the young are hatched. These two groups thus differ very 
markedly from the Ostracoda. The so-called winter eggs 
of the Cladocera are enclosed in a portion of the parental 
test, or in a special covering formed by the parent Cladoceran, 
and then becoming detached within this specialised cover- 
ing or ephippium, they fall to the bottom and remain there 
till the season favourable for their development arrives. 
Of course there are exceptions, and, if the marine species 
belonging to the three groups were included, very peculiar 
exceptions, to the characteristic differences I have tried 
briefly to indicate; but as I am only dealing at present 
with the fresh- and brackish-water species, the exceptions 
are fewer. 
One serious trouble experienced by the student of the 
fresh-water Crustacea, and especially of the Copepoda, is the 
large number that are to be met with in all stages of 
development; and as the penultimate, or antepenultimate, 
stage of one species frequently closely resembles the mature 
stage of another and different species, the greatest vigilance 
is necessary to avoid mistaking the young stage of one. 
species for the adult stage of another; the surest course to 
adopt is to discard all specimens except those where the 
VOL, XII. 2B 
