C. elegans is a nematode, a smooth-skinned worm with a long, unsegmented, cylindrical body tapered at both ends. Comprising about 1,000 cells, it is the most primitive animal to exhibit characteristics that are important in the study of human biology and disease. Though tiny and transparent, C. elegans contains a full set of differentiated tissues, including a nervous system with a "brain," which allows the study of behavior in a worm that is capable of learning. It is found worldwide in soil and rotting vegetation.

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