The water-fleas (_Cyclops_) (fig. 35) are among the smallest of the Crustacea. They are extremely abundant, having great power of multiplication. "An old _Cyclops_ may produce forty or fifty eggs at once, and may give birth to eight or ten broods of children living five to six months. As the young begin to reproduce at an early age, the rate of multiplication is astonishing. The descendants of one _Cyclops_ may number in one year nearly 4,500,000,000, or more than three times the total population of the earth, provided that all the young reach maturity and produce the full number of offspring." The _Cyclops_ feed on smaller aquatic animals such as Protozoa, Rotifera, etc. They in turn serve as food for fishes; and because of their immense numbers and occurrence in all except the swiftest fresh waters "they form the main food of most of our fresh-water fishes while young." Many aquatic insect larvæ feed almost exclusively on them. Related to the _Cyclops_ are a host of other kinds of minute Crustaceans. Among these the so-called fish-lice are specially interesting because of their parasitic habits and greatly modified and degenerate structure. There are many kinds of these parasitic crustaceans infesting fishes, whales, molluscs, and worms. "As on land almost every species of bird or mammal has its own parasitic insects, so in the water almost every species of fish or larger invertebrate has its parasitic crustaceans." Some of the most common of these parasites attach themselves to the gills of fishes. Here they cling, sucking the blood or animal juices from the host. In form of body they do not at all resemble other Crustaceans, but are strangely misshapen. They are often worm-like, or sac-like, without legs or other locomotory appendages. As with other parasites (see Chapter XXX) an inactive dependent life results in the atrophy and loss by degeneration of the body-parts concerned with locomotion and orientation.