ARTHROPODA 259 
split, and is not hollowed out to form a brood-pouch, as is the 
case in the female. 
The testes occupy relatively the same position in the body 
as the ovaries, lying one upon each side of the alimentary canal. 
They are continuous with the vas deferens, which opens by 
means of a muscular ductus ejaculatorius upon the post- 
abdomen. 
The small males are much rarer than the females; they are 
usually to be found in the autumn, but sometimes occur at 
other times of the year, when the conditions of life become un- 
favourable. During the summer the female produces a number 
of summer eggs, which hatch out in the brood-pouch. The 
enormous fertility of the water-fleas is shown by the fact that 
in nineteen days a female D. pulex produced five broods, the 
total number of young being 209; and it has been calculated 
that the descendants of a single individual, which becomes 
mature in ten days, and then produces broods of fifteen young 
every three days, would amount to over twelve million in less 
than two months. The rapidity of developement is rendered 
possible by the nutriment stored up in the summer eggs, and 
this is in some species augmented by the secretion of additional 
food material into the brood-pouch. 
Like many other river animals, such as freshwater Polyzoa 
and Sponges, the Daphnia have developed a means of ensuring 
the existence of the species through the frosts, etc., of winter. 
This is effected by means of the winter eggs. These eggs 
are larger than the summer ones, and contain more 
yolk, a correspondingly large amount of the contents of the 
ovary being absorbed during their maturation. They are in- 
capable of developing without fertilisation. The winter eggs 
pass into the brood-pouch, and a part of the carapace of the 
mother becomes in this region modified to form a capsule for 
the eggs, this is called the ephippiuwm. At the next ecdysis 
the ephippium, which usually contains two eggs, is thrown off 
and floats away. It is a bivalved structure and has a very 
striking resemblance to the statoblasts of some Polyzoa. The 
ephippium may be redeveloped by the female even when 
fertilisation does not occur, but in this case no eggs are laid; 
it may also be replaced by the ordinary brood-pouch. 
