Animals come in a huge range of shapes and sizes, but the vast majority of them are very small. In fact there are many animals that are way smaller than some single-celled organisms.
Here, on the right is a rotifer, an animal with a body made up of around 1,000 cells. At the top, contracting rapidly, is a single-celled organism (Spirostomum spp.). The tip of a needle provides a sense of scale. Disappearing behind the pin at the start of the video is another single-celled organism.