Platyhelminthes, also known as flatworms, are hermaphrodites, meaning individuals of the species produce both sperm and eggs. These are released from the platyhelminthes where they become cross-fertilized by other platyhelminthes. Two types of platyhelminthes, trematoda and cestoda, lay eggs constantly. The turbellarian class has a seasonal cycle. Turbellaria also reproduce asexually by dividing. Turbellaria are the only class able to regenerate body parts.
There are four classes of platyhelminthes: turbellaria, trematoda, cestoda and monogenea. Turbellaria is the only class of platyhelminthes that is not parasitic. Their skin is soft, whereas other classes of platyhelminthes have a scaly exterior. Turbellaria live in water where they feed on other minute creatures. The trematoda class, commonly called flukes, have oral suckers they use to attach to hosts. Cestoda, known as the tapeworm, is a segmented worm without a mouth or digestive system. Each segment, called a proglottid, is devoted to reproduction. Both monogenea and trematoda internal organs are entirely devoted to reproduction, resulting in huge numbers of offspring. One monogenea can produce 100 descendants every three weeks.
All platyhelminthes are flat with long, worm-like bodies. They have no brains, only nerve cords that run the length of the body. Two layers of muscles directly under the skin allow for convulsive movement.