Protozoa inhabit salt water as well as fresh water; they are also found on land in very damp places, and invade animals as parasites.
_Class I._--*Sarcodina* (_Rhizopoda_). Protozoa, the body substance of which forms pseudopodia; many of them are capable of developing chitinous, chalky, or siliceous coverings or skeletal structures, which, however, permit the protrusion of the pseudopodia either over the entire periphery or at certain points. They possess one nucleus or several.
_Order 1._--_Amœbina_ (Lobosa) naked or with a simple shell, sometimes formed of a foreign substance; the pseudopodia may be lobose or finger-shaped; there may be a contractile vacuole; generally only one nucleus. They live in fresh or salt water, in the soil, and also parasitically.
_Order 2._--_Foraminifera_ (Reticularia). Mostly provided with a calcareous shell, usually consisting of several chambers, and allowing the protrusion of the pseudopodia either at the periphery or only at the opening. The pseudopodia are filamentous and frequently anastomosed; there is no contractile vacuole; there are usually several nuclei. Mostly marine.
_Order 3._--_Heliozoa._ Naked, or with a chitinous or simple radial siliceous skeleton; the pseudopodia are filamentous, and are frequently supported by firmer axes, which exhibit no tendency to anastomosis; there is a contractile vacuole; one or several nuclei. Live in fresh water.
_Order 4._--_Radiolaria_. The body has radially-disposed filamentous pseudopodia, and the nucleus is hidden in the central capsule; there is almost always a siliceous framework, consisting of pieces arranged radially, tangentially, or lattice-like; there is no contractile vacuole, but fluid-containing hydrostatic vacuoles are present in the peripheral protoplasm. Marine.
_Class II._--*Mastigophora* (_Flagellata_). Protozoa with one or several long flagella used for locomotion and for acquiring food; in stationary forms their only function is to take in food. Cytostome and contractile vacuole may be present. May be either naked or provided with protective coverings; one or more nuclei. They live either in fresh or salt water, or may be parasitic.
This class is again divided into several sub-classes and orders, of which only the Euflagellata, with the Protomonadina and Polymastigoda are of interest here.
