Planarian, any of several free-living (nonparasitic) worms. The planarian is the simplest of all flatworms. Most planaria are found in freshwater or salt water, but a few live in moist soils on land.

The planarian has a soft, flat, wedge-shaped body that may be black, brown, gray, or white and is about a half inch (1.3 cm) long. The blunt, triangular head has two ocelli (eyespots), pigmented areas that are sensitive to light. There are two auricles (earlike projections) at the base of the head, which are sensitive to touch and the presence of certain chemicals. The mouth is located in the middle of the underside of the body, which is covered with cilia (hairlike projections). The nervous system consists of a simple brain from which two nerve cords extend the length of the body. Other nerves connect these cords, forming a ladderlike structure. There are no circulatory or respiratory systems; oxygen entering and carbon dioxide leaving the planarian's body diffuses through the body wall.

The planarian travels with a gliding motion by moving its cilia. It can also travel short distances by using a rapid crawling motion. The planarian feeds on crustaceans, larvae, and small worms. To eat, it extends a long, tubular pharynx from its mouth. Through this tube it secretes digestive juice onto its prey, then sucks in bits of partly-digested food.

Planaria are hermaphroditic; that is, they possess both male and female sex glands. However, a planarian cannot fertilize its own eggs; the eggs must be fertilized by the sperm of another planarian. In another method of reproduction, a planarian constricts behind the pharynx and separates itself into two pieces (a process called fragmentation); then, both pieces grow back their missing parts (a process called regeneration) and become two new planarians.

The planarianThe planarian can regenerate and become two new planarians.

Scientists have used the planarian for studies on regeneration and for research on learning and behavior. The planarian can learn to respond to a particular stimulus. When such a planarian is cut into several pieces, the new planarians regenerated from the pieces in many cases “remember” the learned response of the original planarian.

Planaria belong to the class Turbellaria of the phylum Platyhelminthes. There are many families and genera.