Planaria - flatworms.
Flatworms are slow moving and thin, and may grow to around 10-15mm in length. They are omnivorous animals feeding partly on prey and partly by scavenging. They occur on the undersides of rocks and debris, in a variety of flow situations. Flatworms move using short microscopic cilia on a path of mucus that gives them a gliding motion. Their body plan features bilateral symmetry, an internal reproductive system, a branching blind gut, an excretory system, and a simple nervous system with an enlarged 'brain' in the front end. Planarians exhibit the ability to regenerate from damage - when cut in half it is possible for both parts to form separate individuals. They avoid strong light and are most active at night. They are the simplest animals to exhibit an ability to learn in response to conditioning.