Characteristics
Planarians are globally distributed and common to many parts of the world, residing in freshwater ponds and rivers. They are also found residing on plants or in marine environments.
Planarians range in size from about 3 to 12 mm. The head has two eyespots (also known as ocelli) that can detect the intensity of light. The eyespots act as photoreceptors and are used to move away from light sources. The head may also have tentacles. The tail is typically pointed.
As with all flatworms, planarians have three germ layers (ectoderm, mesoderm, and endoderm), but lack a true coelom (body cavity).
Planarians have simple organ systems and lack respiratory and circulatory systems. They have a single-opening digestive tract with a three-branched digestive cavity, consisting of one anterior branch and two posterior branches.