46 PHYLUM ARTHROPODA 



Internal Organs. The digestive tract passes from the mouth, 

 which is ventrally placed and lies back of the ventral cleft in the 

 shell, first forward, then turns dorsally and finally posteriorly and 

 extends back to the anus near the end of the abdomen. Near 

 the anterior bend of the digestive tract a pair of colored, curved 

 pouches communicate with it ; they are the livers. The sachke 

 heart may be seen beating rapidly above the intestine. It pos- 

 sesses a pair of lateral openings into which the blood streams 

 from the body cavity with each dilation, and an anterior opening 

 through which it is sent into the forward part of the body with 

 each contraction. There are no other blood vessels. 



The Reproductive Organs. The daphnids which are usually seen 

 are all parthenogenetic females, the males making their appear- 

 ance at certain times of the year only. The female animal is 

 larger than the male, and may be distinguished by its brood sac. 

 This is a large space just beneath the dorsal wall of the thorax 

 in which the eggs and the young brood are carried. The ovaries 

 are a pair of tubular organs alongside the intestine, which com- 

 municate, by means of short oviducts, with the brood sac. The 

 ovaries are easily detected by the presence of large ova in them. 

 These are in groups of four, of which but one, the third, is 

 destined to become an egg, the other three being nutritive cells 

 by which it is nourished. 



During the greater part of the summer the eggs pass into the 

 brood pouch unfertilized and develop there parthenogenetically, 

 producing only females. The young animals pass out of the 

 brood chamber through a posterior opening ; they soon become 

 adult and in their turn give birth to parthenogenetic females. 

 The eggs which thus develop are called summer eggs. At certain 

 times of the year, however, as in the autumn, males are also born. 

 They fertilize the females, and the fertilized eggs then produced 

 differ from those which were unfertilized in possessing thicker 

 shells. They are called winter eggs and are able to resist the cold 

 of winter or the effect of drought. In the spring the winter eggs 

 develop into parthenogenetic females again. 



Exercise 2. Draw an outline of the animal and place in it all the 

 internal organs you have observed. 



