| Description of flatworm |
| distinguishing features |
Platyhelminthes are bilaterally symmetrical animals, their left and right sides are mirror images of each other; this also implies they have distinct top and bottom surfaces and distinct head and tail ends.
* Like other bilaterians, they have three main cell layers (endoderm, mesoderm, and ectoderm), while the radially symmetrical cnidarians and ctenophores (comb jellies) have only two cell layers. Beyond that, they are "defined more by what they do not have than by any particular series of specializations.
* Unlike other bilaterians, Platyhelminthes have no internal body cavity, so are described asacoelomates.