The animal kingdom consists of millions of creatures that live on Earth, and differ among themselves in their composition, shape, and size. They may be so small that they can only be seen through a microscope, or they may be large. Their length may not exceed a few millimeters, or reach more than thirty metres, and some of them live on the highest mountain peaks, or in the depths of the sea, whether in extremely cold regions, such as the North and South Poles, or extremely hot regions, such as the desert region and the equator. Animals of all shapes and sizes live in various parts of the world. Some of them walk or crawl on the ground, some fly in the air or swim in the water, and the animal kingdom is the largest kingdom on the surface of the planet, its subjects number in the millions.