. Elementary textbook of economic zoology and entomology. Zoology; Insect pests. ii4 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. I
. Elementary textbook of economic zoology and entomology. Zoology; Insect pests. ii4 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 ma
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