[This post is adapted from a book I've just finished, which will be published sometime next year by Thames and Hudson.]
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 ciliate, a single-celled organism (Spirostomum spp.). The tip of a needle gives a sense of scale. Disappearing behind the needle at the start of the video is another single-celled organism.
It is creatures like this that really underline the multi-layered complexity of biodiversity. These cilates, swimming around in a small pond, are little bigger than a full stop yet they dwarf the rotifers that crawl and dart around them. Humans on the other hand are big organisms, vastly bigger than most other animal species, but this smaller majority is effectively hidden in plain view, which makes it difficult to appreciate the riot of animal life that surrounds us.