Phylum Platyhelminthes


There are more than 20,000 species of flatworms. The group includes freshwater planarians and colorfully patterned marine flatworms. It also contains parasitic worms that live inside the bodies of an estimated 200 million humans around the world.

Flatworms’ bodies are bilaterally symmetrical and they have a defined head and tail region. They have a central nervous system containing a brain and a nerve cord. Clusters of light-sensitive cells on either side of their head make up what are called eyespots.  The head region of the flatworm has other paired sense organs connected to the flatworm’s simple brain.

Flatworms are both male and female, called hermaphroditic. They are also capable of both sexual and asexual reproduction.  Their bodies have only a single opening, which serves as both a mouth and an anus.


  • Bilaterally symmetrical with a head and a tail
  • Centralized nervous system
  • Three tissue layers
  • No body cavity, no circulatory system and no hard skeleton

Platyhelminthes Fact:

The longest flatworm ever found, a tapeworm, was more than 90 feet long.

Word Bank

Planarian: the simplest flatworm; lives in freshwater
Parasitic: a relationship between species where one species, the parasite, benefits at the expense of the other, the host
Bilaterally symmetrical: the right half is a mirror image of the left side
Hermaphroditic: simultaneously male and female
Asexual: reproduction by splitting apart