184 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



described for Moiiia. The following stages are niuch as described by 

 Grobben. An invagination occurs and a median swelling appears on 

 the ventral aspect of the egg. 



Labrum and second antennre bud out and are soon followed by the 

 antennae, mandibles and two pairs of maxill?e, after which the five 

 pairs of feet soon appear. In an early stage there is present a basal 

 palpus to the second antennae, 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 sac. 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 I), 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 Daplmia 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 



