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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)

Gastrotrich, coloured scanning electron micrograph (SEM). Gastrotrichs are microscopic, worm-like animals found in both freshwater and marine habitats. In total, there are some 400 species of gastrotrichs. This is a freshwater gastrotrich. Its body is covered in cilia (hair-like appendages), which are used for locomotion. It feeds on bacteria, algae and protozoa, living among plants at the margins of ponds and lakes. Magnification: x400 when printed at 10 centimetres wide.

Credit: STEVE GSCHMEISSNER/SCIENCE PHOTO LIBRARY

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