Caenorhabditis elegans (photo above by Bob Goldstein, diagram below by “KDS444”) is a transparent nematode worm, about 1 mm in length. It lives naturally in the soil, where it eats bacteria, but it is also quite happy to make its home in a Petri dish. A 1963 suggestion by Sydney Brenner led to C. elegans becoming the focal point of a vast collaborative effort to understand the worm in detail. Brenner shared the 2002 Nobel Prize in Physiology or Medicine for this work.
The cellular development of C. elegans has been mapped in detail, and its genome had been largely mapped by 1998. The diagram below shows the neural network of the worm, drawn using R, based on data from here (from this paper via this one). In this diagram, colour shows the centrality of neurons in the network. Other information on C. elegans is available at wormbase.org.
Because of the effort that has gone into understanding this humble worm as whole, rather than as just parts, a great deal has been learned about biology in general. Brenner was on to a good thing!