When asked what the world's most numerous animal is, most people assume the insects. The Entomological Society of America claims insects are 1.6 billion times more numerous than humans, in which case there would be about 1019 insects. However, insects, although they are the most diverse animals (with over a million species described by science), are not the most numerous animals — they're too large, usually measuring in the millimeters or centimeters. To find the most numerous animal, we must zoom in to a smaller scale.
Mites, a form of arachnid, not an insect, are ubiquitous in practically every environment, and are often considered one of the most numerous animals on the planet. Like insects, mites are thought to contain over a million different species, but this is even more impressive than in the case of insects. This is because mites (Acarina) are just a subclass of organism, while insects (Insecta) are a Class, a higher taxonomic division. Mites have a smaller average size, around a millimeter millimeters. This tiny size, along with their fast reproductive rates and ubiquitous range, makes it reasonable to estimate that mites outnumber insects at least 10:1. If so, there are more than 1020 mites on Earth.
There are organisms even more numerous than both mites and insects. These include various species of microfauna, most notably the nematodes, also known as roundworms. Nematodes are very small (many have about 1000 cells, and measure 1-2 mm in length) and very numerous. They cover practically every square inch of the land and sea. Nematodes account for 90% of all life on the ocean floor. In certain fertile areas of topsoil, there are over a trillion nematodes per square mile. Most biologists agree that nematodes are both the most ubiquitous and most numerous animal on Earth. It is reasonable to estimate that nematodes outnumber mites 10:1, and probably greater than 100:1. If so, then there are 1021 - 1022 nematodes on Earth, making them the most numerous animal.