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: