The Platyhelminthes, also known as flatworms, are divided into four major classifications that are the Turbellaria, Trematoda, Monogenea, and Cestoda. They are the simplest of worm groups and contain thousands of species. They can be found mostly in marine and fresh water. One of the most common types of Platyhelminthes is the tape worm which can get into the host's digestive tract and eat the food that they eat, preventing the nutrients in that food from getting into the host. The nervous system of the Platyhelminthes involves two chords of nerves that go down either side of its body. It contains a simple brain that is called ganglia and is made up of bundles of nerves. It also has two eyes that help it sense light. The nervous system is concentrated in the head, but are connected to nerve trunks all around the body. What the nervous system does in the Platyhelminthes is detect light, touch, water movement, and chemical stimuli that help with the detection of food. Their eyes help them sense light around them and help them move and interact with their environment. Their pointed lobes make them sensitive to touch and help them avoid things that could possibly be predators. They have three layers called Endodern, Mesoderm, and Ectoern that maintain homeostasis within their body. The nervous system within the Platyhelminthes are mostly simple, but some can be complex.