ii 4 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



of the changes that may take place in them when the density of 

 the water is changed. Artemia and several other genera be- 

 longing to this order form a very important food supply for 

 many fishes. 



To the order Ostracoda belong several genera; Cypris, which 

 occurs in fresh water, and Cypridina, which is marine, are the 

 most common. These resemble minute bivalve shells and 

 often occur in great numbers near the surface where the 

 surface-haunting fish and other animals feed on them. 



In the order Copepoda the body is 

 long and distinctly segmented, and the 

 appendages are confined to the head 

 and thorax. The water-fleas, Cyclops, 

 are common in all pools or quiet waters. 

 The single median eye has suggested 

 the generic name. These are often used 

 as examples to show how rapidly some 

 of these forms may multiply. It has 

 been estimated that the descendants 

 of a single individual might in one 

 year number nearly 4,500,000,000, if all 

 the young lived and produced their full 

 number of offspring. They feed on 

 other smaller animals such as Protozoa 

 and Rotifera, and in turn serve as one 

 of the most important sources of food 

 for the young of many fresh water fishes. Related to Cyclops 

 is the genus Cetockilus which occurs in the surface water of 

 the sea in inconceivable numbers, sometimes forming almost a 

 solid mass extending for miles in quiet waters. Those whales 

 which are furnished with fringes of whalebone in the mouth 

 often swim in schools through the water where these minute 

 creatures abound, and as the water rushes through the wide- 

 open mouth the minute crustaceans are strained out and 

 thus furnish a dainty and abundant supply of food for the 

 largest of living animals. To the genus Sapphirina belong 

 some of the most wonderfully phosphorescent animals that 

 are found in the sea. This order also includes the so-called 



FIG. 43. Water-flea, 

 Cyclops, female with 

 egg masses. (From life, 

 greatly magnified.) 



