The animal that has the fewest neurons and is therefore a candidate for being the least intelligent is Caenorhabditis elegans, a nematode worm that lives in the soil. This nematode is the first animal to have its nervous system reconstructed and its neural pathways mapped.
C. elegans is essentially a tube with a mouth, and its genome is relatively simple. It can eat E. coli and consists of approximately 1,000 cells.
Some species of jellyfish have as few as 800 neurons, whereas mice have approximately 71 million in their nervous systems. For comparison, humans have approximately 86 trillion neurons, over a million times as many as a mouse.