Planarians
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 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.