To answer this question, we need to first understand what is an animal. According to Wikipedia,
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan becomes fixed as they develop, usually early on in their development as embryos, although some undergo a process of metamorphosis later on in their life.
The answer to the question of "smallest animals" by San Diego Zoo suggests protozoans, which belong to planktonic animals. Protozoans are often referred to as one-celled animals whose characteristics are found in both plants and animals. You need a microscope to see protozoans.
This is echoed in Joshph Agro's post to answer the same question,
Here is what my friend's opinion on which animal group would have to be considered: "I think protozoans would be considered the smallest animal, EXCEPT that it appears they are no longer considered animalia. I'd guess the next smallest would be from the insect kingdom (which has so many uncharted species finding the smallest would be difficult), or perhaps the sponge. That's my guess."
The major argument is that
Until the 1970s the general view was that the protozoans were animals, and as such they were placed in the animal kingdom as the phylum Protozoa.
Botanists, on the other hand, classified the same organisms as algae, which were regarded as plants.
Even for protozoans, there are more than 65,000 described species, of which over half are fossil.
You might find that there maybe no scientific answer to the question of world's smallest animal. As pointed out by Jim,
It's much easier to find the biggest animal in the world, than the smallest.
There are more different kinds of ultra-tiny animals than there are normal big animals. No matter what I find, there's probably a smaller one somewhere that I haven't seen yet.
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