The simplest animals, which are called the protozoa, consist of single cells. Every animal except a few which reproduce by splitting or budding, starts life as a single cell. And fossils of protozoa are found in pre-Cambrian rocks before anything bur fragmentary fossils of other groups are found. A group of them, called the trvpanosomes, are one of the curses of Africa. They cause 'sleeping sickness' in man, and a number of diseases in animals. Amoeba is too small to see clearly without a microscope, but some other rhizopods are quite large, and build complicated skeletons of limestone or silica. A symmetrical animal swimming blind will go in a straight line or a circle, so it will either leave the favourable area quickly, or come back to where it started. An asymmetrical one moves in a corkscrew spiral and can visit most parts of a small volume of water in the course of time.