The next class which may be known as the whip-bearers (_Mastigophora_) includes those Protozoa that move by fine undulating processes called flagella. One of the common representatives of this class is the little green _Euglena_ (Fig. 9), whose presence in standing ponds and puddles often imparts a greenish color to the water. Then in the salt water near the surface there are often myriads of minute _Noctiluca_ whose wonderfully phosphorescent little bodies glow like coals of fire when the water is disturbed at night. Although this class contains fewer forms than the preceding some of these have within recent years been found to be of great importance because they live as parasites on man and other animals. The trypanosome whose presence in the blood and tissues of the patient causes that dreadful disease which ends in sleeping sickness belongs here as well as do several other similar kinds that produce serious troubles for various mammals and birds. The Spirochæta, about which there has been so much recent discussion, also belong here. These are simple spiral-like forms (Fig. 10), that are sometimes classed with the simple plants, bacteria, but Nuttall and others have shown very definitely that they should be classed with the simplest animals, the Protozoans. These are the cause of relapsing fevers in man and of several diseases of domestic animals. It is believed by certain eminent zoölogists that when the germ that causes yellow fever is discovered it will be found to belong to this group. The members of the class Infusoria, so called because they were early found to be abundant in various infusions, are characterized by numerous fine cilia or hair-like organs by means of which the organism moves about and procures its food. The well-known "slipper animalcule" (_Paramoecium_) (Fig. 11), and the "bell-animalcule" (_Vorticella_) (Fig. 12) are two common representatives. The _Paramoecia_ were the animals mostly used by Jennings in his wonderfully interesting experiments on the behavior of these lowly forms of life. He showed that they always reacted in a certain definite way in response to particular stimuli, and he was led to believe that he could see "what must be considered the beginnings of intelligence and of many other qualities found in the higher animals." A species of _Vorticella_ was probably the first Protozoan that was ever observed. An old Dutch microscopist, Anton von Leeuwenhoek, in 1675, while studying with lenses of his own manufacture, discovered and described forms which undoubtedly belong to this genus. Few if any of the Infusoria are pathogenic, although some are said to be associated with certain intestinal diseases both in man and the lower animals (Fig. 13). [Illustration: FIG. 11--_Paramoecium._ (From Kellogg's Elementary Zoöl.)] [Illustration: FIG. 12--_Vorticella_, one individual with the stalk coiled, the other with the stalk extended. (From Kellogg's Elementary Zoöl.)]