184 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
described for Moina. The following stages are much as described by 
Grobben. An invagination occurs and a median swelling appears on 
the ventral aspect of the egg. 
Labrum and second antenne bud out and are soon followed by the 
antennie, mandibles and two pairs of maxille, after which the five 
pairs of feet soon appear. In an early stage there is present a basal 
palpus to the second antennz, a fact not before observed, and this per- 
sists as the small two-bristled wart found on the basal joint of the an- 
tenna. It is a conspicuous object in the embryo and is thus a true 
embryonic organ. 
The eyes of the embryo appear as two separate pigmented flecks 
which approximate and are covered with an oval refractive body, 
which later is penetrated by the pigment and divides to form the 
small lenses. Soon after this the shell grows over the eye as described 
for Moina. 
The first indication of the shell appears as two folds of the maxil- 
lary region of the back, being thickest laterally. These grow forward 
and backward to form the cephalic and body shield. At a little later 
stage there appears a very interesting modification of the shell which 
stands in close relation to the growth of the brood sae. A slight pro- 
tuberance appears on the margin of the shell in the median dorsal 
line and extends toward the abdomen. It grows much more rapidly 
than the other parts of the shell and, in a later stage, forms a compara- 
tively enormous tail, which curves under the animal between the 
shell valves which now extend beyond the body. This ‘‘tail’’ extends 
well along the ventral margin of the shell and reminds, by its posi- 
tion, of the tail of a frightened dog. The true tail, or post-abdomen, 
is, in the meanwhile, well developed and is constantly kicking the 
useless protuberance of the shell upwards. As the animal leaves the 
egg this projection becomes straightened as in the young D. pulex, 
finally becoming the still considerable spine, though it is proportion- 
ately much shorter than in the embryo. The spine becomes shorter 
with successive moults, and the mature form has only a slight rounded 
knob in place of a spine more than half the length of the body. 
The use of the long spine in the young Daphnia is a matter of in- 
terest. Its length agrees pretty well with that of the brood cavity 
and it seems possible that it serves to prevent the shell from bending 
abruptly down when it is only partially removed during the moult 
and thus breaking off and so leaving a portion of the clothing of the 
brood cavity therein to become a source of irritation. This is more 
necessary for the young, since the brood cavity is narrow and the 
shell weak, so that while the outer shell is removed like a glove from 
the finger, it cannot be pulled upward or downward, but directly 
