Fig. 149.—Two species of Proteus Animalcule (Amœba), one with protoplasmic processes, slender and branched; the other with short blunt processes. Highly magnified.
This sub-kingdom includes animals which are mostly microscopic, or at any rate small, and live in the sea, fresh water, or damp places. They are unicellular, and consist of a soft viscous substance (protoplasm or sarcode) of contractile nature, and therefore capable of altering its shape. They may or may not be enclosed by a firm wall. Many secrete siliceous structures or calcareous shells. They are of no agricultural importance. Here belong: Infusoria, common in putrid solutions; and Rhizopods, such as Amœba, the Proteus Animalcule (Fig. 149), so called because its shape is constantly altering.