- nematodes divided into either 5 or 12 clades
- crown clades - (III-V or 8-12) - most well studied nematodes belong here (including c elegans)
- all appear to have simple invariant nervous systems
- other clades have much more variable numbers of neurons
- crown clades - (III-V or 8-12) - most well studied nematodes belong here (including c elegans)