[This answer][1] mentions that the C. elegans hermaphrodite has [exactly 302 distinct neurons][2]. This has made it a very effective model for a variety of types of biological research, including neurology and cell differentiation. It is also currently the only organism with a completely mapped [connectome][3].

But the word "always" made me wonder - has a viable specimen ever been verified to naturally have a number of neurons *other than 302*? Not as a result of an experiment, just naturally?

[1]: http://biology.stackexchange.com/a/57595/27918
[2]: https://en.wikipedia.org/wiki/Caenorhabditis_elegans#Research_use
[3]: https://en.wikipedia.org/wiki/Connectome