The New Student's Reference Work/Protozoa

Protozoa (prō′tō̇-zō′ȧ), the simplest animals. Collectively, they form the sub-kingdom of protozoa. They are microscopic and unicellular, and abound in stagnant water. They are of great interest to zoologists and physiologists, because in them the processes of life are reduced to their simplest expression. By studying them many facts are obtained that help in understanding the more complex animals. They have recently been studied in reference to their reactions to stimuli, and many important results obtained. The daily life of a protozoön was made the subject of study in one of our laboratories of physiology. The group includes simple forms like the amœba and its relatives, having root-like feet and constituting the class of Rhizopoda. Many of the rhizopods have no covering, but others either secrete a shell or gather hard particles and build one. Chalk is largely made of minute, limy shells of protozoa. The form most common in chalk is a cluster of globular chambers, varying in size. The walls of these shells are perforated by minute holes, through which the soft protoplasm of the animal protrudes. They constitute an order called foraminifera or hole-borers. These animals are abundant in warm and temperate seas. They live near the surface and, as they die, their shells sink and reach the bottom. Deep-sea explorations have brought from great depths a soft ooze, formed almost entirely of these shells of foraminifera. Somewhat similar forms make shells of great beauty, of quartz or silica, which cannot be dissolved by acid or melted by fire. They constitute the order of Radiolaria. The higher protozoa belong mainly to the class of Infusoria (which see). These minute animals have hair-like processes (cilia), which propel the body through the water like the action of many small oars. In some forms the hair-like threads are large, reduced in number to one or two and called flagella. Thus the class of infusoria is separated into the two divisions of the Flagellata and the Ciliata. The latter contains the highest developed of the protozoa. Among the more common forms may be noted the slipper-animalcule (Paramœcium), with a free-swimming, slipper-shaped body; the bell-animalcule (Vorticella), with a bell-shaped body anchored by a stalk; and the trumpet-animalcule or stentor. The rhizopoda and the infusoria are the chief classes; two others of less importance are recognized. See Leidy's Freshwater Rhizopods of North America, beautifully illustrated, published by the United States government; and Kent's Manual of the Infusoria.