58 INVERTEBRATE ZOOLOGY 
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. In the male the testes 
occupy a position similar to that of the ovaries. Their external 
openings are on the ventral side of the abdomen. 
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 cer- 
tain 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 springtime the winter eggs develop into parthenogenetic 
females again. 
Bxercise 3. Draw an outline of the animal and place in it all 
the internal organs you have observed. 
