Flatworms type Platyhelmintes;

A group of invertebrate animals that are flattened and bilaterally symmetrical. Their shape usually resembles a leaf, a plate or a tape. Unlike Ctenophora or Cnidaria, flatworms have three types of cells: ectoderm, mesoderm and gastroderm. They have digestive (with the exception of some parasitic flatworms living also in humans), excretory, reproductive and nervous systems but lack circulatory and respiratory ones.

Flatworms are hermaphroditic with complex reproductive systems. Female systems produce huge number of eggs.

Flatworms are often parasitic. An example of parasitic flatworms living in fish is Trematoda (there are also Trematoda living in humans).