The mnC1 homozygote on the left is paralyzed from the "neck" down due to an unc-52 mutation on chromosome II. This "balancer" strain is used to map mutations to the region of chromosome II. I made the animated GIF by using NIH image software. For more information on balancers in C. elegans see the Genetic Balancers chapter of "Caenorhabditus elegans: Modern Biological Analysis of an Organism" Methods in Cell Biology, Volume 48 Academic Press.