ENTOMOSTRACA OF MINNESOTA. 185 
backwards. It is well known that male Daphniw often have the spine 
while the females may have none, and here again it is possible that 
the narrower cavity over the abdomen requires this assistance, while 
this is not the case with the females. 
The shell gland is early formed and the branchial lamelle of the 
feet appear almost simultaneously with the feet themselves as distinct 
lobes. The branchial chamber is not a simple chamber, but is essen- 
tially a curved tube, as can be very well seen in the last foot of the 
adult. This tube doubles upon itself and crosses in the manner of a 
loop and a constant stream flows rapidly through it. 
The nervous system is, at first, paired from beginning to end and 
first unites anteriorly, the ocular ganglia fuse after the union of the 
two pigment flecks in the compound eye, then the cephalic ganglion 
is formed by the union of the two precesophageal ganglia, and the com- 
missures passing about the cesophagus. I have not been able to de- 
termine if the subcesophageal ganglia become fused. From the anterior 
ganglia spring the nerves to the antenne and jaws, which latter are © 
larger in the embryo. 
It is greatly to be regretted that no sufficient revision of Daphnia 
can now be attempted. This is partly due to the very imperfect 
nature of the descriptions and partly to the scattered literature of 
the group. We here supply nothing more than a provisional key 
and synoptical descriptions. I take pleasure in acknowledging the 
substantial assistance, in respect to recent literature of the group, 
received from my friend, Professor Birge, as also for valuable sugges- 
tions respecting Cladocera in general. 
KEY TO THE GENUS DAPHNIA. 
I. Pigment spot present. 
A. Anal margin of the post-abdomen deeply excavated. . . Schaefferi, 187 
B. Anal margin slightly if at all sinuous. 
a. Head very short and regularly curved from the heart to the 
beak. . . . . . . . . +. ~»psittacea, 187; atkinsonii, 188 
b. Head not very short or regularly curved. 
* Claws of post-abdomen with a distinct comb of spines. 
t A strong toothed process from the dorsal margin of 
the head at some time of life. 
ii Spinelonsstraisht. . . . 3) ... 4 . elathrata, 188 
{i Spine short, curved. . . . . . . . minnehaha, 189 
tt No strongly toothed dorsal process at any period in 
either sex. 
{ Head with a?helmet-like crest. 
2 Spine long, springing from the dorsal angle, 
Curnvedydorsad an pete cunrewoey rae ne 8S VExiiS S190 
