48 
POPULAR SCIENCE REVIEW. 
shell, which covers it like a buckler, but opens interiorly, to give 
issue to the antennse, organs of the mouth and feet. 
In this water-flea, as in the other, a distinguishing mark is 
the single large eye on the forehead, which in some of the 
species is of a fine ruby colour. The large antennae, or horns, 
as they were wont to be called by the earlier writers, are four 
in number, the two upper of which are long and generally com- 
posed of numerous joints or articulations, furnished with short 
hairs or setae. Immediately below the antennae is situate the 
mouth, and a little lower down we see a pair of organs called by 
some the foot-jaws , and by others the hands. Descending in 
the body, we next see the feet, which are five pairs in number, 
and are each double, or composed of two stalks arising from a 
common base, each stalk consisting generally of from two to four 
joints, more or less furnished with hairs. These feet, in this 
species of entomostracon, are solely adapted for the act of 
swimming, instead of serving, as in the preceding, for respira- 
tion. In the female (fig. 11), the young are not hatched in 
the internal ovary, as in the daphnia, but the ova are very 
soon passed into an external ovary, which forms a little bag, 
projecting on each side of the tail, and remain suspended there 
for several days. In this external ovarian bag the ova remain 
till they are hatched, when the sac bursts open, and the fry are 
ushered forth into life. But how unlike the parent do these 
young ones appear! The earlier observers of these newly-born 
animals could scarcely believe their own eyes. The celebrated 
microscopist Leeuwenhoek, amongst the first to notice this 
fact, was so surprised at the unexpected discovery that it was 
not till after repeated experiments that he became satisfied with 
the result, and was persuaded that these little creatures, so 
different in appearance from the parent, were actually born from 
the very eggs which he had been so patiently watching. 
Immediately after birth, the young animal presents the 
appearance represented in fig. 13, a form so very different from 
that of the adult that Muller actually could not believe his pre- 
decessors who had witnessed the birth, but placed it in a distinct 
genus ! It is extremely curious and interesting to watch the 
transforming process which these little creatures undergo. It 
takes about twenty days from the birth, more or less, according 
to the heat of the weather, before they have become perfect 
animals. The body gradually becomes elongated ; segmentation 
takes place (fig. 14) ; the feet by degrees take on their proper 
form, till at length the little creature throws off its shell, or 
moults, and assumes, though not completely developed, the shape 
and appearance of the adult (fig. 15). After the third moulting, 
it is perfect and capable of reproducing its species. The process 
of moulting is equally curious and interesting in the four-horned 
