Planarians are flatworms known for their ability to regenerate lost body parts. Their unique feeding mechanism offers insight into their biology and how they interact with their environment. Understanding how these creatures locate, capture, and process food reveals adaptations that support their survival.
Locating and Securing Food
Planarians primarily locate food through chemoreception, using specialized sensory structures called auricles. These ear-like projections on the sides of their head are densely packed with chemoreceptors, allowing them to detect dissolved chemicals, such as amino acids and proteins, released by potential prey or decaying organic matter. This chemical sensitivity enables them to follow concentration gradients, effectively “smelling” their way toward a food source. Planarians are also sensitive to water disturbances, which can indicate the presence of live prey.
Once a food source is detected, planarians secure their meal. They are carnivorous scavengers that feed on small invertebrates like insect larvae, crustaceans, and other worms, as well as decaying organic material. They often crawl over their prey, gripping it with their anterior end, and may secrete a sticky mucus from their body surface. This mucus helps to trap smaller organisms or immobilize larger ones.
Ingestion Through the Pharynx
Planarian ingestion involves a muscular, tube-like organ called the pharynx. The planarian’s mouth is located on its underside, typically near the middle of its body. When a planarian positions itself over its food, the pharynx can be rapidly extended outwards through this opening.
The extended pharynx acts like a straw or a vacuum cleaner, attaching to the food source. Muscular contractions within the pharynx create a suction force, drawing food particles into the planarian’s digestive system. This muscular tube is capable of flexible movements, including elongation, shortening, and bending, allowing the planarian to manipulate and ingest its meal. The pharynx is composed mainly of muscle cells, enabling this precise feeding action.
Digestion and Nutrient Distribution
After ingestion, food enters the planarian’s gastrovascular cavity, a branched network that serves both digestive and circulatory functions. This cavity extends throughout much of the planarian’s body. Its extensive branching provides a large surface area for nutrient absorption and ensures efficient distribution of digested food.
Digestion begins with extracellular processes where enzymes break down food into smaller particles. These partially digested particles are engulfed by specialized cells, called phagocytes, that line the gastrovascular cavity. Intracellular digestion then occurs within food vacuoles inside these phagocytic cells, completing nutrient breakdown. Nutrients are absorbed by these cells and distributed throughout the body through the fluid within the branched cavity, effectively acting as a primitive circulatory system. Undigested waste material is expelled back out through the pharyngeal opening.