Caenorhabditis elegans, often shortened to C. elegans, is a minuscule, transparent roundworm, approximately 1 millimeter in length. This unsegmented nematode serves as a widely recognized model organism in biological research due to its simplicity and the visibility of its internal structures. Its transparency allows for direct observation of its development and cellular activities.
Overall Body Plan and Outer Protection
The body plan of C. elegans exhibits bilateral symmetry. Internally, it possesses a pseudocoelom, a fluid-filled body cavity not fully lined by mesoderm. This space provides hydrostatic support and allows for organ movement. Simple, longitudinal muscles running along its body facilitate the worm’s characteristic sinusoidal locomotion.
A tough, flexible outer layer called the cuticle, composed primarily of collagen, protects the worm externally. This acellular layer provides structural support and acts as a barrier against environmental stressors and pathogens. As C. elegans grows, it sheds its old cuticle through molting, typically occurring four times during its larval development, allowing for increases in body size.
The Digestive Pathway
The digestive system of C. elegans is a complete, linear tube extending from the mouth to the anus. Food intake begins with the pharynx, a muscular pumping organ in the head region. This structure uses rhythmic contractions to grind and ingest bacteria, its primary food source, and acts as a gatekeeper, filtering out larger particles.
From the pharynx, food passes into the intestine, which runs almost the entire length of the worm’s body. Here, digestive enzymes break down food particles, and nutrients are absorbed into the pseudocoelomic fluid. The intestine is a primary site for nutrient assimilation. Undigested waste then moves into the rectum.
Waste is expelled from the body through the anus, a small opening near the tail. This unidirectional flow ensures efficient processing and nutrient extraction.
Nervous System and Sensory Organs
The nervous system of C. elegans, though simpler than that of vertebrates, operates on conserved principles of neuronal organization. It consists of a central nerve ring, often referred to as the worm’s “brain,” which encircles the pharynx in the head region. This nerve ring integrates sensory information and coordinates motor responses. Major nerve cords, specifically the ventral and dorsal cords, extend along the length of the body from the nerve ring, connecting to muscles and other neurons.
Numerous sensory neurons are distributed throughout the worm’s body, allowing it to perceive its environment. In the head, specialized chemosensory organs called amphids detect chemical cues, such as food sources and pheromones, guiding the worm’s foraging and reproductive behaviors. These amphid neurons are critical for the worm’s ability to navigate its surroundings. Similarly, phasmids, which are chemosensory organs located in the tail, also contribute to chemical sensing and play a role in avoidance behaviors.
The entire nervous system comprises a precise and invariant number of neurons, with the adult hermaphrodite having 302 neurons and the male having 385 neurons. This neuronal constancy has made C. elegans an invaluable model for mapping neural circuits and understanding how specific neurons contribute to behavior. The well-defined connections between these neurons have been extensively mapped, providing a comprehensive “connectome” for the organism.
Reproductive and Excretory Systems
C. elegans primarily reproduces as a self-fertilizing hermaphrodite, meaning each individual possesses both male and female reproductive organs. The hermaphrodite reproductive system includes a U-shaped gonad that produces both sperm and oocytes. These gametes mature and are stored until fertilization occurs internally. The fertilized eggs then pass through the uterus and are laid externally through the vulva, a ventral opening.
While hermaphrodites are the predominant form, males, which possess only testes and spicules for transferring sperm, also exist in smaller populations. Males are capable of mating with hermaphrodites, allowing for genetic outcrossing.
The excretory system of C. elegans is relatively simple, focusing primarily on osmoregulation and waste removal. It consists of a large, H-shaped excretory cell located in the head region. This cell extends canals that run along the length of the body, collecting metabolic waste products and regulating internal fluid balance. Waste is then expelled from the body through a small excretory pore located on the ventral side, near the pharynx.
References
WormBook: The Online Review of C. elegans Biology. The Nervous System. Accessed August 22, 2025.
WormAtlas. Excretory System. Accessed August 22, 2025.