Here's why:
* Bilateral Symmetry: This means an organism can be divided into two mirror-image halves along a central axis. This is evident in nematodes as they have a distinct head and tail end, a dorsal (back) and ventral (belly) side, and a left and right side.
* Radial Symmetry: This means an organism can be divided into identical parts around a central axis, like a pie. This is not the case with nematodes.
Think of it this way: you can cut a nematode in half lengthwise and get two similar halves. You can't do this with a starfish, which has radial symmetry.