How small can an animal get?
This organism is a gastrotrich, an invertebrate found in the sediment of aquatic environments. They can be as small as .06 mm, or about the size of 10 human red blood cells laid flat end to end. And keep in mind that red blood cells are smaller than most of our cells!
Despite their size, these multi-cellular animals come with their own complete digestive system, nervous system, reproductive organs, and muscle tissue. They may not be the smallest (some rotifers and obligate parasites are smaller), but they certainly push the limit for how small an animal can be.
(Photos are mine, information source is Animal Earth by Ross Piper and this)