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!?, ^ulex 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 ephippium. 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. 



