Soil nematode about 1mm long that lives in temperate regions. Doesn't really have a common name... except maybe 'tiny worm' or something generic like that. A favorite animal model for biological research because so much is known about it. In fact, we know an adult C. elegans has exactly 959 somatic cells, and their full lineage all the way back to the zygote. We're pretty close to sequencing the critter's entire genome. Another reason C. elegans is so easy to study is that it has a transparent body and each of its cells can be examined individually under a microscope without dissecting it. Mutations have been found in C. elegans that slow down its metabolic rate and at the same time increase its lifespan.
You can find out more about C. elegans than any reasonable person would ever want to know at this site specifically devoted to that one animal:

Caenorhabiditis elegans is a free-living (non-parasitic) species of soil nematode which makes a good model organism for biological study because it has a small genome of only six chromosomes. It also has a short generation time of about three days (at room temperature), and is easy to grow at high densities (up to 10,000 worms on one Petri dish).

C. elegans has been thoroughly studied by geneticists, developmental biologists and neurologists. The worms can be used to study genetic manipulation, gene therapy, and the molecular basis of differentiation during embryonic development. Much of the world's knowledge about aging, genetic inheritance, and the factors that control gene expression during development comes from studying this and other nematodes.

The full taxonomic classification of C. elegans is:

Kingdom Animalia
Phylum Nematoda
Class Secernentea
Subclass Rhabditia
Order Rhabditida
Family Rhabditidae
Genus Caenorhabiditis
Species Elegans

From the science dictionary at

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