Caenorhabditis elegans
C. elegans is a nematode about 1 mm in length, and lives in the soil, especially rotting vegetation, in many parts of the world. Its average life span is about 2-3 weeks.
It has a nervous system with a ‘brain’ (the circumpharyngeal nerve ring). It exhibits behavior and is even capable of rudimentary learning. Neural structures include sensing organs in the head which responses to taste, smell, temperature and touch—and although C. elegans has no eyes, it can respond slightly to light.