Platyhelminthes are small worms with flattened body plans and simple organs of seemingly ancient origin. Individual species in this group are called flatworms or tapeworms, indicating their distinct body shape.
Platyhelminthes share a few other characteristics that make them unique. Their bodies are bilaterally symmetrical along the horizontal, meaning that when their bodies are folded, the left and right edges touched, making the two edges look the same. This is considered an evolutionary advancement over older phylums, which are radially symmetrical. Radial symmetry is when the body can be folded over like segments of a circle. One example of an animal with radial symmetry is the starfish.
Platyhelminthes have simple eye spots for detecting light, organs that are considered an early evolutionary form of more complex animal phylum's eyes. They possess a nervous system of branching nerves that run straight down the length of the body, with a distinct mass of nerves near their eye spots. This mass is a kind of primitive brain. Their bodies are made up of three distinct layers, another innovation that has been passed down to later phylums. Many species possess a digestive cavity, but none has a digestive system with an anus, so they have to excrete waste through their mouths.
Most animals in the phylum are parasites, living inside larger animals and feeding off their hosts. However, the group Turbellaria live outside of hosts in salt and fresh water. Platyhelminthes reproduce sexually, but individual worms possess both kinds of reproductive organs. Most species have developed adaptations to avoid fertilizing themselves with their own gametes. Some species can also reproduce asexually.