Caenorhabditis elegans
Caenorhabditis elegans is a small, unsegmented, transparent member of the order Rhabditida in the phylum Nematoda (Nemata). Measuring approximately 1 millimeter in length in its adult form, this roundworm is typically found in soil, often feeding on bacteria among rotting vegetation and decaying organic matter, and is nonparasitic (free living) and nonpathogenic. Despite being a primitive invertebrate, C. elegans possesses many of the anatomical features and organs found in higher-level animals, such as those pertaining to its nervous, reproductive, muscular, and digestive systems; however, it lacks a circulatory system and a respiratory system. Because C. elegans is transparent, easily observed and manipulated, has a relatively short life cycle (2–3 weeks), and can be cultivated in large numbers under laboratory conditions (with 10,000 nematodes per petri dish), researchers frequently use it to study questions that would be difficult to explore directly in humans and other animals. In particular, because the cells of C. elegans hold steady in number and position in the body throughout its life, it is a popular model organism in developmental biology, embryology, genetics, and neuroscience. See also: Developmental biology; Developmental genetics; Embryology; Genetics; Invertebrate embryology; Nemata (Nematoda); Nervous system (invertebrate); Rhabditida