Platyhelminthes

A large phylum of animals, called flatworms (Platyhelminthes is Greek for flatworm). Flatworms are among the simplest of the bilaterally symmetric animals. Tissues are grouped into organs, and there is a complete gut (except in acoel flatworms), but there are no respiratory or circulatory systems, and the animals lack a coelom.

As flatworms are primarily defined by a lack of characteristics with respect to the other triploblastic animals, it is quite possible that they form a polyphyletic group. Three classes of flatworms are generally considered to be closely related, however:

• Monogonatea?
• Cestoda? (flukes)
• Trematoda? (tapeworms)

The last two are parasitic groups, some of which are extremely damaging to people and other animals.