216 



THE SCIENTIFIC FEEDING OF ANIMALS 



hatch into females, which again lay fertile summer eggs (Fig. 93). This 

 process of unisexual reproduction (parthenogenesis) continues during 

 the entire summer. In the fall of the year the summer eggs develop 

 into males and females. The female now develops one large winter 

 egg in its dorsal brood organ (Fig 94), which requires fertilization 

 before it can develop. The winter egg is protected by a hard shell 

 against external influences. It remains dormant throughout the winter 

 and is not aflfected by freezing temperatures. It seems, in fact, as 

 though the exposure to the cold of winter gives additional vigor and 

 power of reproduction. This is one of the beneficial effects of draining 

 fishponds for the winter. 



Figs. 93 and 94. Water flea {Daphnia pulex.) 

 a. Female with summer eggs; b, female with one winter egg. 



Feed for Fish 



Water fleas live on unicellular plant and animal life and prefer warm, 

 shallow ponds rich in organic matter. They often occur in enormous 

 numbers and impart a distinct color to the water. 



b\ Cyclops or hoppers. — Jurine has estimated that a single pair can 

 produce 45,000,000,000 offspring in a single season. They feed and 

 develop in a manner similar to that of the daphnids. 



c^. Ostracoidea. — These are entomostracans with hard bi-valve 

 shells. Their habits, etc., are similar to those of the cyclopses. 



d^ Amphipoda or sand fleas. — Numerous species of various sizes 

 and habits. Those in question are characteristic of this form of life 

 in pure cold water. 



